Add a quick how to for magit-gh-pull in CONTRIBUTING.org

CONTRIBUTING.org

@@ -30,6 +30,7 @@ Thanks! :heart: :heart: :heart:
   - [[#contributing-a-keybinding][Contributing a keybinding]]
   - [[#contributing-a-banner][Contributing a banner]]
 - [[#reviewing-pull-requests][Reviewing Pull Requests]]
+  - [[#using-magit-to-quickly-test-prs][Using Magit to quickly test PRs]]
 - [[#additional-information][Additional information]]
   - [[#testing][Testing]]
 - [[#credits][Credits]]
@@ -261,6 +262,34 @@ judgement to either merge the PR or require further review from another
 reviewer. This is done to ensure a thorough cross-referencing in case of complex
 changes, your review is very valuable in these cases as well!
 
+** Using Magit to quickly test PRs
+It is possible to manage PRs directly inside the Magit status buffer ~SPC g s~.
+First add the =github= layer to your dotfile which will pull the package
+=magit-gh-pulls=. Since we have a high number of PRs in Spacemacs you also need
+to set the variable =magit-gh-pulls-pull-detail-limit= to a higher number, it
+is recommended to set it to something like 200.
+
+#+BEGIN_SRC elisp
+(setq-default dotspacemacs-configuration-layers
+  '((github :variables magit-gh-pulls-pull-detail-limit 200)))
+#+END_SRC
+
+You can also set the variable in your =dotspacemacs/user-init= function.
+
+Now you are all set open the Magit status buffer of your =.emacs.d= repository
+and press ~#g~. It will fetch all the PRs which may take a few seconds as we
+have lot of PRs. Note also that all your Magit actions will get some additional
+delay due to the refresh of the PRs list.
+
+Search for the line of the PR you are interested in and press ~#f~ to actually
+fetch its commits.
+
+From here you have several possibilities:
+- you can select all the commits of a PR and do ~AA RET~ to cherry-pick them
+  on top of your current branche,
+- you can create a branch pointing on the PR branch HEAD using ~#b~,
+- you can merge the PR branch with ~#m~.
+
 * Additional information
 ** Testing
 Tests live in the =tests/= folder, with a folder structure corresponding to the