Dev-Dep guide: mention AST third party requirements

Avoid the likes of:

```
Error:
ActiveStorage::RepresentationTest#test_representing_an_MP4_video:
Errno::ENOENT: No such file or directory - ffmpeg
    test/models/representation_test.rb:27:in `block in <class:RepresentationTest>'
```

Also note additiona macOS requirements in ActiveStorage guide.

[ci skip]

1 files changed, 28 insertions, 0 deletions

diff --git a/guides/source/development_dependencies_install.md b/guides/source/development_dependencies_install.md
index 50274d700b..fdd560ccf3 100644
--- a/guides/source/development_dependencies_install.md
+++ b/guides/source/development_dependencies_install.md
@@ -376,3 +376,31 @@ command inside of the `activestorage` directory to install the dependencies:
 ```bash
 yarn install
 ```
+
+Extracting previews, tested in ActiveStorage's test suite requires third-party
+applications, `FFmpeg` for video and `muPDF` for PDFs, and on macOS also
+`XQuartz` and `Poppler`.. Without these applications installed, ActiveStorage
+tests will raise errors.
+
+On macOS you can run:
+
+```bash
+brew install ffmpeg
+brew cask install xquartz
+brew install mupdf-tools
+brew install poppler
+```
+
+On Ubuntu, you can run:
+
+```bash
+sudo apt-get update && install ffmpeg
+sudo apt-get update && install mupdf mupdf-tools
+```
+
+On Fedora or CentOS, just run:
+
+```bash
+sudo yum install ffmpeg
+sudo yum install mupdf
+```