dev: Document using the merge bot (#166)

* dev: Document using the merge bot

dev/mergebot.rst

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+Interacting with the Merge Bot
+==============================
+
+There is a bot that hangs around on the Syncthing Github projects and
+assists with doing correct merges of pull requests. Using the merge bot to
+accept pull requests is the recommended way in all cases as it enforces some
+extra checks that the "standard" Github merge button does not. The merge bot
+is currently called ``st-review``.
+
+Merging a PR
+------------
+
+To merge a pull request, simply tell the bot to do so, making sure that the
+first word of the command is ``merge``::
+
+    @st-review merge
+
+.. image:: merge-1.png
+
+It's also possible to override the resulting commit subject and message when
+doing this. Just add a blank line, the commit subject, another blank line,
+and then the commit body (which can be empty). Don't worry about the text
+formatting - the commit body will be reflowed appropriately by the bot::
+
+    @st-review merge
+
+    lib/dialer: Add env var to disable proxy fallback (fixes #3006)
+
+.. image:: merge-2.png
+
+Handling Check Results
+----------------------
+
+The merge bot will wait for status checks to resolve, but will refuse to
+merge pull requests with unclean statuses:
+
+.. image:: merge-3.png
+
+It is possible to override this in cases where it's necessary, by adding a
+``Skip-check`` command to the commit message body. Note that this must be in
+the commit message *body*, which means that you need to supply both a commit
+message subject and body. Don't overuse this -- it's better to ask Jenkins
+to retest if something spurious happened. It can be used to allow merge of
+commits from unregistered authors that only touches comments, for example.
+
+The tag must be exactly ``Skip-check:`` followed by a *space separated* list
+of check "contexts" as seen in the list on Github. I.e., to skip these two
+checks:
+
+.. image:: merge-4-0.png
+
+Use the following syntax::
+
+    @st-review merge this please
+
+    all: Correct spelling in comments
+
+    Skip-check: authors pr-build-mac
+
+.. image:: merge-4-1.png
+
+Please note that the exact string ``Skip-check: authors`` is magic in that
+it also allows the build to pass, when it would otherwise stop with commits
+from unknown authors.