Add some build instructions to README.
Add some build instructions to README.

saketkumar95/zulip-android,abhaymaniyar/zulip-android,zulip/zulip-android,Sam1301/zulip-android,prati0100/zulip-android,prati0100/zulip-android,Sam1301/zulip-android,saketkumar95/zulip-android,abhaymaniyar/zulip-android,vishwesh3/zulip-android