README: Add Discord invite link

Add invite link to Discord per announcement email at
https://lore.kernel.org/openbmc/20201112145740.GB4495@heinlein/

Signed-off-by: Patrick Williams <patrick@stwcx.xyz>
Change-Id: Iedde67dc94d071206688bece588ac2615bbb9f01
diff --git a/README.md b/README.md
index 3098f84..d31eafd 100644
--- a/README.md
+++ b/README.md
@@ -174,5 +174,6 @@
 
 ## Contact
 - Mail: openbmc@lists.ozlabs.org [https://lists.ozlabs.org/listinfo/openbmc](https://lists.ozlabs.org/listinfo/openbmc)
+- Discord: https://discord.gg/69Km47zH98
 - IRC: #openbmc on freenode.net
 - Riot: [#openbmc:matrix.org](https://riot.im/app/#/room/#openbmc:matrix.org)