chore: document useful git commands to avoid any DCO trouble

Signed-off-by: Himanshu Mishra <[email protected]>

.github/PULL_REQUEST_TEMPLATE.md

@@ -11,3 +11,4 @@
 - [ ] Added or updated documentation
 - [ ] Tests for new functionality and regression tests for bug fixes
 - [ ] Screenshots attached (for UI changes)
+- [ ] All your commits have a `Signed-off-by` line in the message. ([more info](https://github.com/backstage/backstage/blob/master/CONTRIBUTING.md#developer-certificate-of-origin))

.github/styles/vocab.txt

@@ -65,6 +65,7 @@ Protobuf
 Proxying
 Raghunandan
 Readme
+rebase
 Recharts
 Redash
 Repo

CONTRIBUTING.md

@@ -96,6 +96,12 @@ Awesome commit message
 Signed-off-by: Jane Smith [email protected]
 ```
 
+- In case you forgot to add it to the most recent commit, use `git commit --amend --signoff`
+- In case you forgot to add it to the last N commits in your branch, use `git rebase --signoff HEAD~N` and replace N with the number of new commits you created in your branch.
+- If you have a very deep branch with a lot of commits, run `git rebase -i --signoff $(git merge-base -a master HEAD)`, double check to make sense of the commits (keep all lines as `pick`) and save and close the editor. This should bulk sign all the commits in your PR. Do be careful though. If you have a complex flow with a lot of branching and re-merging of work branches and stuff, merge-base may not be the right solution for you.
+
+Note: If you have already pushed you branch to a remote, you might have to force push: `git push -f` after the rebase.
+
 ## Creating Changesets
 
 We use [changesets](https://github.com/atlassian/changesets) to help us prepare releases. They help us make sure that every package affected by a change gets a proper version number and an entry in its `CHANGELOG.md`. To make the process of generating releases easy, it helps when contributors include changesets with their pull requests.

README.md

@@ -46,7 +46,7 @@ Check out [the documentation](https://backstage.io/docs/getting-started) on how
 ## Community
 
 - [Discord chatroom](https://discord.gg/MUpMjP2) - Get support or discuss the project
-- [Good First Issues](https://github.com/backstage/backstage/contribute) - Start here if you want to contribute
+- [Contributing to Backstage](https://github.com/backstage/backstage/blob/master/CONTRIBUTING.md) - Start here if you want to contribute
 - [RFCs](https://github.com/backstage/backstage/labels/rfc) - Help shape the technical direction
 - [FAQ](https://backstage.io/docs/FAQ) - Frequently Asked Questions
 - [Code of Conduct](CODE_OF_CONDUCT.md) - This is how we roll