Documentation - Provide contributing dev instructions on using forks

- Add details on how to use a fork of the Wagtail code and installing it
- Based on https://stackoverflow.com/a/57407742/8070948
- Closes #10594

CHANGELOG.txt

@@ -5,6 +5,7 @@ Changelog
 ~~~~~~~~~~~~~~~~
 
  * Refine wording of page & collection privacy using password is a shared password and should not be used for secure content (Rohit Sharma, Jake Howard)
+ * Add contributing development documentation on how to work with a fork of Wagtail (Nix Asteri, Dan Braghis)
  * Maintenance: Remove duplicate 'path' in default_exclude_fields_in_copy (ramchandra-st)
 
 

CONTRIBUTORS.md

@@ -790,6 +790,7 @@
 * Andreas Donig
 * Osaf AliSayed
 * ramchandra-st
+* Nix Asteri
 
 ## Translators
 

docs/contributing/developing.md

@@ -345,3 +345,21 @@ pre-commit install
 ```
 
 pre-commit should now run on every commit you make.
+
+(developing_using_a_fork)
+
+## Using forks for installation
+
+Sometimes it may be necessary to install Wagtail from a fork. For example your site depends on a bug fix that is currently waiting for review, and you cannot afford waiting for a new release.
+
+The Wagtail release process includes steps for static asset building and translations updated which means you cannot update your requirements file to point a particular git commit in the main repository.
+
+To install from your fork, from the root of your Wagtail git checkout (and assuming the tooling for building the static assets has previously been installed using `npm install`), run:
+
+```sh
+python ./setup.py sdist
+```
+
+This will create a `.tar.gz` package within `dist/,` which can be installed with `pip`.
+
+For remote deployments, it's usually most convenient to upload this to a public URL somewhere and place that URL in your project's requirements in place of the standard `wagtail` line.