Introduce a benchmark template [ci skip]

This replaces boilerplate in the “benchmark your code” section of the
contributors’ guide with an executable template. I also amended the text
to encourage best practices and codified it in the template.

For now this is only good for relatively self-contained changes that can
be inlined into a simple script. In the future, this can be expanded to
cover how to measure the difference between two commits.

The output looks like this:

```
==================================== Empty =====================================

Warming up --------------------------------------
              blank?   225.963k i/100ms
         fast_blank?   238.147k i/100ms
Calculating -------------------------------------
              blank?      8.825M (± 6.4%) i/s -     44.063M in   5.014824s
         fast_blank?      9.311M (± 6.3%) i/s -     46.439M in   5.009153s

Comparison:
         fast_blank?:  9310694.8 i/s
              blank?:  8824801.7 i/s - same-ish: difference falls within error


================================= Single Space =================================

Warming up --------------------------------------
              blank?    56.581k i/100ms
         fast_blank?   232.774k i/100ms
Calculating -------------------------------------
              blank?    813.985k (±16.7%) i/s -      4.017M in   5.076576s
         fast_blank?      9.547M (± 5.2%) i/s -     47.719M in   5.013204s

Comparison:
         fast_blank?:  9547414.0 i/s
              blank?:   813985.0 i/s - 11.73x  slower


================================== Two Spaces ==================================

Warming up --------------------------------------
              blank?    58.265k i/100ms
         fast_blank?   244.056k i/100ms
Calculating -------------------------------------
              blank?    823.343k (±16.2%) i/s -      4.020M in   5.014213s
         fast_blank?      9.484M (± 4.9%) i/s -     47.347M in   5.005339s

Comparison:
         fast_blank?:  9484021.6 i/s
              blank?:   823343.1 i/s - 11.52x  slower


=============================== Mixed Whitspaces ===============================

Warming up --------------------------------------
              blank?    53.919k i/100ms
         fast_blank?   237.103k i/100ms
Calculating -------------------------------------
              blank?    763.435k (±16.8%) i/s -      3.720M in   5.018029s
         fast_blank?      9.672M (± 5.8%) i/s -     48.369M in   5.019356s

Comparison:
         fast_blank?:  9672467.2 i/s
              blank?:   763435.4 i/s - 12.67x  slower


=============================== Very Long String ===============================

Warming up --------------------------------------
              blank?    34.037k i/100ms
         fast_blank?   240.366k i/100ms
Calculating -------------------------------------
              blank?    409.731k (± 8.9%) i/s -      2.042M in   5.028235s
         fast_blank?      9.794M (± 4.3%) i/s -     49.035M in   5.016328s

Comparison:
         fast_blank?:  9794225.2 i/s
              blank?:   409731.4 i/s - 23.90x  slower
```

1 files changed, 18 insertions, 27 deletions

diff --git a/guides/source/contributing_to_ruby_on_rails.md b/guides/source/contributing_to_ruby_on_rails.md
index 4f938f5deb..5791e7a12a 100644
--- a/guides/source/contributing_to_ruby_on_rails.md
+++ b/guides/source/contributing_to_ruby_on_rails.md
@@ -270,33 +270,24 @@ The above are guidelines - please use your best judgment in using them.
 
 ### Benchmark Your Code
 
-If your change has an impact on the performance of Rails, please use the
-[benchmark-ips](https://github.com/evanphx/benchmark-ips) gem to provide
-benchmark results for comparison.
-
-Here's an example of using benchmark-ips:
-
-```ruby
-require 'benchmark/ips'
-
-Benchmark.ips do |x|
-  x.report('addition') { 1 + 2 }
-  x.report('addition with send') { 1.send(:+, 2) }
-end
-```
-
-This will generate a report with the following information:
-
-```
-Calculating -------------------------------------
-            addition   132.013k i/100ms
-  addition with send   125.413k i/100ms
--------------------------------------------------
-            addition      9.677M (± 1.7%) i/s -     48.449M
-  addition with send      6.794M (± 1.1%) i/s -     33.987M
-```
-
-Please see the benchmark/ips [README](https://github.com/evanphx/benchmark-ips/blob/master/README.md) for more information.
+For changes that might have an impact on performance, please benchmark your
+code and measure the impact. Please share the benchmark script you used as well
+as the results. You should consider including these information in your commit
+message, which allows future contributors to easily verify your findings and
+determine if they are still relevant. (For example, future optimizations in the
+Ruby VM might render certain optimizations unnecessary.)
+
+It is very easy to make an optimization that improves performance for a
+specific scenario you care about but regresses on other common cases.
+Therefore, you should test your change against a list of representative
+scenarios. Ideally, they should be based on real-world scenarios extracted
+from production applications.
+
+You can use the [benchmark template](https://github.com/rails/rails/blob/master/guides/bug_report_templates/benchmark.rb)
+as a starting point. It includes the boilerplate code to setup a benchmark
+using the [benchmark-ips](https://github.com/evanphx/benchmark-ips) gem. The
+template is designed for testing relatively self-contained changes that can be
+inlined into the script.
 
 ### Running Tests