Add more old per-project docs to our general git usage

tools/git-usage.md

@@ -1,4 +1,24 @@
-# Git Usage
+# Git usage
+
+## Getting in sync with other developers
+
+To bring in changes from other developers into the branch you are
+working on, from main `git pull` and then `git checkout your-branch-name` and `git rebase main`
+
+## Git troubleshooting
+
+If you've accidentally made commits to main and cannot pull in other's changes as a result.
+
+To get rid of a commit (the most recent one),
+
+```
+git reset HEAD^
+```
+
+to then be able to `git stash` changes from the commit, now local changes.  To just throw everything in the commit away, `git reset --hard HEAD^`
+
+See https://sethrobertson.github.io/GitFixUm/fixup.html
+
 
 ## When NOT to manually resolve merge conflicts