Add funding link to github mirror

see https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/displaying-a-sponsor-button-in-your-repository

Change-Id: I2ceef868ae52c52329413a7caeaa3ca00a40b983
diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml
new file mode 100644
index 0000000..7592deb
--- /dev/null
+++ b/.github/FUNDING.yml
@@ -0,0 +1 @@
+open_collective: osmocom