CONTRIBUTING: Added instructions for PR workflow

.github/CONTRIBUTING.md

@@ -43,18 +43,36 @@ You can (and should) run our test suite using [*tox*].
 However, you’ll probably want a more traditional environment as well.
 We highly recommend to develop using the latest Python release because we try to take advantage of modern features whenever possible.
 
-Clone the *attrs* repository to your computer:
+First [fork](https://github.com/python-attrs/attrs/fork) the repository on GitHub.
+
+Clone the fork to your computer:
 
 ```console
-$ git clone git@github.com:python-attrs/attrs.git
+$ git clone git@github.com:<your-username>/attrs.git
 ```
 
 Or if you prefer to use Git via HTTPS:
 
 ```console
-$ git clone https://github.com/python-attrs/attrs.git
+$ git clone https://github.com/<your-username>/attrs.git
+```
+
+Then add the *attrs* repository as *upstream* remote:
+
+```console
+$ git remote add upstream git@github.com:python-attrs/attrs.git
 ```
 
+You can of course also rename the *origin* remote (your fork) to *fork* and add the repository as *origin*.
+
+The next step is to sync the upstream repository with your local copy:
+
+```console
+$ git fetch upstream
+```
+
+This is important to obtain eventually missing tags, which are needed to install the development version later on. See #1104
+
 Change into the newly created directory and after activating a virtual environment install an editable version of *attrs* along with its tests and docs requirements:
 
 ```console
@@ -81,6 +99,16 @@ $ make html
 
 The built documentation can then be found in `docs/_build/html/`.
 
+To file a pull request, create a new branch on top of the upstream repository:
+
+```console
+git checkout -b my_topical_branch upstream/main
+```
+
+Make your changes, push them to your fork (the remote *origin*) and publish the PR!
+
+To sync the *attrs* repository with your local copy, just pull from the *upstream* remote.
+
 ---
 
 To avoid committing code that violates our style guide, we strongly advise you to install [*pre-commit*] and its hooks:

changelog.d/1105.change.md

@@ -0,0 +1 @@
+Added instructions for pull request workflow to CHANGELOG.md