add sponsor button on gitHub repo

GitLab: https://git.jami.net/savoirfairelinux/jami-client-android/-/issues/1459
Change-Id: I21a7e8ac8227c95b8bd467fe0d161bac903a4e78
diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml
new file mode 100644
index 0000000..a9636fa
--- /dev/null
+++ b/.github/FUNDING.yml
@@ -0,0 +1,2 @@
+custom: ['https://jami.net/whydonate/']
+lfx_crowdfunding: jami-a-gnu-package