Update Adding Content steps to use GitHub Issue setup

[strixy16]

Which content needs to be updated?
The current version of the Adding Content page under Contributing doesn't include the creating an issue step. So if someone makes a branch, it might not automatically associate with a branch unless they understand the naming convention on the commits/PRs.

Describe the update
Make the first step read:

1. Select or Create an Issue

Navigate to the Issues section of the repo and select an existing issue or continue on from the Issue you created in the Submitting Issues section.
Assign yourself to the issue if it's not already assigned.
Open the issue and click on the "Create a branch" link under Development on the right side of the page.
Follow this naming convention (same as what's there right now)
Run the lines of code provided (git fetch, git checkout branchname)

With this change, step 4 can also be updated since users won't need to run the set-upstream part anymore, so it will become

git add .
git commit -m "Add new getting started page"
git push 

Reason for Update
We want to ensure every branch is associated with an existing issue so conversation and documentation can be followed properly.