Add open_collective to FUNDING.yml

Announcement:
https://medium.com/airbnb-engineering/continuing-our-support-of-lottie-open-source-community-9e50e66ebb04

Documentation:
https://docs.github.com/en/free-pro-team@latest/github/administering-a-repository/displaying-a-sponsor-button-in-your-repository

.github/FUNDING.yml

diff --git a/.github/FUNDING.yml b/.github/FUNDING.yml
new file mode 100644
index 0000000..a1e5821
--- /dev/null
+++ b/.github/FUNDING.yml
@@ -0,0 +1 @@
+open_collective: lottie