[gnome-build-meta/mcatanzaro/readme] Add instructions for building a local runtime

[Michael Catanzaro <mcatanzaro src gnome org>]

commit 0dbf93fd9b36efb1afc38517f4524219349076f5
Author: Michael Catanzaro <mcatanzaro gnome org>
Date:   Fri Mar 26 19:38:52 2021 +0000

    Add instructions for building a local runtime

 README.rst | 9 +++++++++
 1 file changed, 9 insertions(+)
---
diff --git a/README.rst b/README.rst
index ff72831c..729ee696 100644
--- a/README.rst
+++ b/README.rst
@@ -5,3 +5,12 @@ build metadata for building the GNOME software stack.
 
 The content of this repository is a `BuildStream <https://wiki.gnome.org/Projects/BuildStream>`_
 project. Instructions for building GNOME can be `found here 
<https://wiki.gnome.org/Newcomers/BuildSystemComponent>`_.
+
+To build a runtime locally, for debugging:
+
+```
+$ bst build flatpak-runtimes.bst
+$ bst checkout flatpak-runtimes.bst repo
+$ flatpak remote-add --user --no-gpg-verify testrepo repo
+$ flatpak install testrepo org.gnome.Platform
+```