readme: Add GitHub sponsors badge in README.
readme: Add GitHub sponsors badge in README.

The number is really low, since we haven't told anyone about our
GitHub Sponsors site, so the badge probably isn't actually helpful,
but I suppose that's OK.

kou/zulip,eeshangarg/zulip,punchagan/zulip,rht/zulip,kou/zulip,eeshangarg/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,zulip/zulip,andersk/zulip,kou/zulip,andersk/zulip,eeshangarg/zulip,rht/zulip,kou/zulip,hackerkid/zulip,punchagan/zulip,rht/zulip,punchagan/zulip,zulip/zulip,andersk/zulip,hackerkid/zulip,punchagan/zulip,punchagan/zulip,eeshangarg/zulip,kou/zulip,eeshangarg/zulip,eeshangarg/zulip,andersk/zulip,andersk/zulip,rht/zulip,zulip/zulip,rht/zulip,andersk/zulip,zulip/zulip,kou/zulip,zulip/zulip,hackerkid/zulip,zulip/zulip,kou/zulip,eeshangarg/zulip,rht/zulip,punchagan/zulip,hackerkid/zulip,hackerkid/zulip