[gi-docgen] Add MANIFEST.in file for setuptools

[Emmanuele Bassi <ebassi src gnome org>]

commit c3b10d8dc518ffd50470ff89c9cfa202805df0fa
Author: Emmanuele Bassi <ebassi gnome org>
Date:   Thu Apr 15 14:53:16 2021 +0100

    Add MANIFEST.in file for setuptools
    
    Downstream distributors use the source distribution, instead of the
    built one; this means we need to add all files to the manifest.

 MANIFEST.in | 4 ++++
 1 file changed, 4 insertions(+)
---
diff --git a/MANIFEST.in b/MANIFEST.in
new file mode 100644
index 0000000..73e6a41
--- /dev/null
+++ b/MANIFEST.in
@@ -0,0 +1,4 @@
+include README.md meson.build gi-docgen.pc.in
+include gi-docgen.py
+include .flake8
+recursive-include gidocgen/templates *