Add submitting a patchset documentation

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docs/Submitting a patchset.md

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+# Submitting a patchset
+
+## About patchsets
+
+On sourcehut, rather than using merge requests, people contribute code
+by emailing a patchset of their changes to a public mailing list.
+
+A patchset is a set of changes in text form that can be applied as
+commits to a git repository.
+
+## git-send-email.io
+
+This article takes information from [https://git-send-email.io][gse], which
+includes an interactive tutorial which allows you to practise sending
+patchsets. If you'd like to practise, you should go there now and follow
+the tutorial. Otherwise, keep reading this article to learn what you
+need to do for Bibliogram specifically.
+
+[gse]: https://git-send-email.io
+
+## Submitting a patchset to Bibliogram
+
+(You'll need to first configure git for email as described on
+git-send-email.io.)
+
+If you haven't yet, clone the repository that you want to edit. Here's
+how you clone the main Bibliogram repository: `git clone
+https://git.sr.ht/~cadence/bibliogram`
+
+Set the default email address: `git config sendemail.to
+'~cadence/bibliogram-devel@lists.sr.ht`
+
+Create a topic branch to work on: `git checkout -b my-cool-changes`
+
+Make the changes you want to make using a text editor.
+
+Add and commit your changes to the topic branch.
+
+Email the patch to the mailing list: `git send-email master`
+
+(`master` here represents which commits you want to send. Everything
+_after_ the specified revision will be sent, so `master` will send
+everything on your topic branch. You can also try `HEAD^` to send the
+latest commit, `HEAD^3` to send the latest 3, or `abcdef1` to send
+everything after a specific commit.)
+
+If you're asked to make changes, it's okay to use `git commit --amend`
+and `git rebase` if you wish to. When you've made the changes, use `git
+send-email -v2 master`. If you have to go back for a third attempt, use
+`-v3`, and so on.
+
+If you run into problems, you can ask for help [on matrix] or by
+composing an email to `~cadence/bibliogram-devel@lists.sr.ht`.
+
+[on matrix]: https://matrix.to/#/#bibliogram:matrix.org