DOC: Add documentation for editing another persons PR

Author: Kai-Striega

doc/source/dev/checking_out_an_upstream_pr.md

@@ -0,0 +1,49 @@
+# Editing another person's pull request
+
+## Respect
+
+Please be respectful of other's work.
+
+## Expected setup
+
+This guide expects that you have set up your git environment as is outlined in [getting_the_code](getting_the_code.md). 
+In particular, it assumes that you have:
+
+1. a remote called ``origin`` for your fork of NumPy-Financial
+2. a remote called ``upstream`` for the original fork of NumPy-Financial
+
+You can check this by running: 
+
+```shell
+git remote -v
+```
+
+Which should output lines similar to the below:
+
+```
+origin	https://github.com/<your_username>/numpy-financial.git (fetch)
+origin	https://github.com/<your_username>/numpy-financial.git (push)
+upstream	https://github.com/numpy/numpy-financial.git (fetch)
+upstream	https://github.com/numpy/numpy-financial.git (push)
+```
+
+## Accessing the pull request
+
+You will need to find the pull request ID from the pull request you are looking at. Then you can fetch the pull request and create a branch in the process by:
+
+```shell
+git fetch upstream pull/<ID>/head:<BRANCH_NAME>
+```
+
+Where:
+
+* ``<ID>`` is the id that you found from the pull request
+* ``<BRANCH_NAME>`` is the name that you would like to give to the branch once it is created.
+
+Note that the branch name can be anything you want, however it has to be unique.
+
+## Switching to the new branch
+
+```shell
+git switch <BRANCH_NAME>
+```

doc/source/dev/getting_the_code.md

@@ -22,6 +22,55 @@ git clone https://github.com/<your_username>/numpy-financial.git
 
 Hooray! You now have a working copy of NumPy-Financial.
 
+## Adding the upstream repo
+
+Now that your fork of NumPy-Financial is available locally, it is worth adding the upstream repository as a remote. 
+
+You can view the current remotes by running: 
+
+```shell
+git remote -v
+```
+
+This should produce some output similar to:
+
+```shell
+origin	https://github.com/<your_username>/numpy-financial.git (fetch)
+origin	https://github.com/<your_username>/numpy-financial.git (push)
+```
+
+Now tell git that there is a remote repository that we will call ``upstream`` pointing to the numpy-financial repository: 
+
+```shell
+git remote add upstream https://github.com/numpy/numpy-financial.git
+```
+
+We can now check the remotes again:
+
+```shell
+git remote -v
+```
+
+which gives two additional lines as output:
+
+```shell
+origin	https://github.com/<your_username>/numpy-financial.git (fetch)
+origin	https://github.com/<your_username>/numpy-financial.git (push)
+upstream	https://github.com/numpy/numpy-financial.git (fetch)
+upstream	https://github.com/numpy/numpy-financial.git (push)
+```
+
+
+## Pulling from upstream by default
+
+We want to be able to get the changes from the upstream repo by default. This way you pull the most recent changes into your repo.
+
+To set up your repository to read from the remote that we called `upstream`:
+
+```shell
+git config branch.main.remote upstream
+git config branch.main.merge refs/heads/main
+```
 
 ## Updating the code with other's changes