hacking: Add a section on how to upgrade the bundled Scintilla copy

HACKING

@@ -677,6 +677,22 @@ When you have these two files, you have to list your new test along the
 other ones in the ``test_source`` variable in ``tests/ctags/Makefile.am``.
 Please keep this list sorted alphabetically.
 
+Upgrading Scintilla
+-------------------
+
+To upgrade the local Scintilla copy, use the ``scripts/update-scintilla.sh``
+script.
+
+To use it, you need to first obtain a copy of the Scintilla sources you want
+to update to.  This will generally mean checking out a release tag from the
+Scintilla Mercurial repository, or extracting a tarball.
+
+Then, just run the script from Geany's to source directory passing the path
+to the Scintilla source directory as first argument, and follow the
+instructions, if any::
+
+    ./scripts/update-scintilla.sh /path/to/scintilla/
+
 GDB
 ---