doc: add info on syncing to origin/main and rebasing (#1540)

Author: terriko

.github/actions/spelling/allow.txt

@@ -14,6 +14,7 @@ apk
 Args
 asn
 asottile
+atlassian
 autoescape
 autoextract
 autoextracts
@@ -286,6 +287,7 @@ Rahul
 readme
 readthedocs
 realpython
+rebasing
 refactoring
 regex
 Romi

CONTRIBUTING.md

@@ -230,6 +230,13 @@ git pull
 git checkout -b my_new_branch
 ```
 
+>Note: If you accidentally check something in to main and want to reset it to match our main branch, you can save your work using `checkout -b` and then do a `git reset` to fix it:
+>```
+>git checkout -b saved_branch
+>git reset --hard origin/main
+>```
+>You do not need to do the `checkout` step if you don't want to save the changes you made.
+
 When you're ready to share that branch to make a pull request, make sure you've checked in all the files you're working on.  You can get a list of the files you modified using `git status` and see what modifications you made using `git diff`
 
 Use `git add FILENAME` to add the files you want to put in your pull request, and use `git commit` to check them in.  Try to use [a clear commit message](https://chris.beams.io/posts/git-commit/) and use the [Conventional Commits](https://www.conventionalcommits.org/) format.  
@@ -279,6 +286,8 @@ Someone will review your code and try to provide feedback in the comments on Git
 
 If something needs fixing or we have questions, we'll work back and forth with you to get that sorted.  We usually do most of the chatting directly in the pull request comments on GitHub, but if you're stuck you can also stop by our [Gitter chat room](https://gitter.im/cve-bin-tool/community) to talk with folk outside of the bug.
 
+>Another useful tool is `git rebase`, which allows you to change the "base" that your code uses.  We most often use it as `git rebase origin/main` which can be useful if a change in the main tree is affecting your code's ability to merge.  Rebasing is a bit much for an intro document, but [there's a git rebase tutorial here](https://www.atlassian.com/git/tutorials/rewriting-history/git-rebase) that you may find useful if it comes up.
+
 Once any issues are resolved, we'll merge your code.  Yay!
 
 In rare cases, the code won't work for us and we'll let you know.  Sometimes this happens because someone else has already submitted a fix for the same bug, (Issues marked [good first issue](https://github.com/intel/cve-bin-tool/labels/good%20first%20issue) can be in high demand!) or because you worked on a checker that didn't have a good signature. Don't worry, these things happens, no one thinks less of you for trying!