Add README.md and USE-BRANCH-main-NOT-master hint

* README.md is a copy of <https://github.com/EDCD/EDMarketConnector/wiki/Git-Using-Main-Branch>.
* USE-BRANCH-main-NOT-master is empty and just another hint.

README.md

@@ -0,0 +1,37 @@
+# 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.
+
+## Migrating your own project from 'master' to 'main'
+
+We followed the instructions at https://www.hanselman.com/blog/EasilyRenameYourGitDefaultBranchFromMasterToMain.aspx
+, which boil down to:
+
+1. `git checkout master`
+1. `git fetch origin`
+1. `git branch -m master main`
+1. `git push -u origin main`
+