Add undoing an accidental merge

Git-Basics.md

@@ -93,3 +93,36 @@ $ git rebase master
 $ git push origin cider
 ```
 And now your `cider` branch is updated onto the changes present in the updated master branch.
+
+## Best Practices: Undoing an accidental merge
+At some point you probably will accidentally merge the main repo into your branch. 
+You will need to undo this; we cannot accept MRs that have a merge as a commit.
+The sooner you fix it the better.
+
+Let's say you have accidentally merged changes from the main repo into a branch caller `cider` on your fork.
+Here's how you can undo it:
+
+0) If you are worried you may lose some work copy the files somewhere else. 
+In theory you should not lose anything but if you do not feel confident with git copy your changes elsewhere.
+
+1) Roll your branch back to before you merged (ideally just one commit back):
+```
+git reset --hard HEAD~1
+```
+
+or
+
+```
+git reset --hard {commit hash}
+```
+
+2) Force push this rollback to *your* repo/branch (`cider` in this example)
+
+```
+git push --force origin cider
+```
+
+3) Follow [Best Practices: Updating Your Fork](Git-Basics.md?ref_type=heads#best-practices-updating-your-fork).
+
+4) If you've lost something you did not backup you can use ```git checkout {hash}``` to try and find the pieces.
+