doc: add a nice explainer about how to update PR branches

This is good info to have, that I often forget and have to spend time
looking for. It'd be nice to have it documented in the project somewhere
so people new the project (or new to maintaining maybe?) know where to
find it.

doc/contribute-to-core-lightning/contributor-workflow.md

@@ -91,6 +91,66 @@ extract-bolt-csv`. By default the bolts will be retrieved from the directory `..
 
 e.g., `make extract-bolt-csv BOLTDIR=../bolts BOLTVERSION=ee76043271f79f45b3392e629fd35e47f1268dc8`
 
+
+## Pushing Up Changes to PR Branches
+
+If you want to pull down and run changes to a PR branch, you can use the convenient
+pr/<pr#> branch tags to do this. First you'll need to make sure you have the following
+in your `.github/config`.
+
+	[remote "origin"]
+	    fetch = +refs/pull/*/head:refs/remotes/origin/pr/*
+
+Once that's added, run `git fetch` and then you should be able to check out PRs by their number.
+
+	git checkout pr/<pr#>
+
+
+If you make changes, here's how to push them back to the original PR originator's
+branch. NOTE: This assumes they have turned on "allow maintainers to push changes".
+
+First you'll want to make sure that their remote is added to your local git. You
+can do this with `remote -v` which lists all current remotes.
+
+	git remote -v
+
+If it's not there, you can add it with
+
+	git remote add <name> <repo_url>
+
+For example, here's how you'd add `niftynei`'s git lightning clone.
+
+	git remote add niftynei git@github.com:niftynei/lightning.git
+
+
+To push changes to the remote, from a `pr/<pr#>` branch, you'll need to
+know the name of the branch on their repo that made the PR originally. You
+can find this on the PR on github.
+
+You'll also need to make sure you've got a ref to that branch from their repo;
+you can do this by fetching the latest branches for them with the following command.
+
+	git fetch niftynei
+
+You may need to fetch their latest set of commits before pushing yours, you can do
+this with
+
+	git pull -r niftynei <pr-branch/name>
+
+Finally, you're good to go in terms of pushing up the latest commits that you've made
+(or changed) on their branch.
+
+	git push <name> HEAD:<pr-branch/name>
+
+For example, here's how you'd push changes to a branch named "nifty/add-remote-to-readme".
+
+	git push niftynei HEAD:nifty/add-remote-to-readme
+
+
+If that fails, go check with the PR submitter that they have the ability to push changes
+to their PR turned on. Also make sure you're on the right branch before you push!
+
+
 ## Release checklist
 
 Here's a checklist for the release process.