Add instructions for using 'main' as primary branch

Git-Using-Main-Branch.md

@@ -0,0 +1,26 @@
+# Using 'main' as the primary branch in Git
+
+This project uses 'main', instead of 'master', for its primary branch.
+
+If you had previously cloned with 'master' as the primary then you
+should follow these steps to migrate:
+
+1. `git checkout master`
+1. `git branch -m master main`
+1. `git fetch`
+1. `git branch --unset-upstream`
+1. `git branch -u origin/main`
+1. `git symbolic-ref refs/remotes/origin/HEAD refs/remotes/origin/main`
+
+These steps are:
+
+1.    Go to the master branch
+1.    Rename master to main locally
+1.    Get the latest commits from the server
+1.    Remove the link to origin/master
+1.    Add a link to origin/main
+1.    Update the default branch to be origin/main
+
+See
+[Contributing.md](https://github.com/EDCD/EDMarketConnector/blob/main/Contributing.md)
+for an outline of the other branches we use.