templates: Add macro for appending branches to webhook URLs.

This macro is an alternative to git-webhook-url-with-branches.md,
which contains the full URL with a `branches` query parameter at
the end. This macro is for when we just want to specify that this
can be done but the URL to append to can be variable or is unique
to a particular integration (and thus, doesn't warrant its own
macro and a full URL example).

templates/zerver/help/include/git-append-branches.md

@@ -0,0 +1,4 @@
+You can also limit the notifications you receive to specific branches
+by appending `?branches=master,development` to the end of the URL,
+where `master` and `development` are the branches you'd like to be
+notified about.