Update README.md

Author: Aversefun

README.md

@@ -1,2 +1,15 @@
-# x3139-chess
-xkcd 3139 chess
+# X3139 Chess
+
+15 sliding puzzle, but chess! Inspired by [xkcd 3139](https://xkcd.com/3139). Currently supports most chess features, including:
+- Castling
+- En passant
+- Showing available moves
+- Requiring you to get out of check if you are in check
+
+Stuff that will be implemented Soon :tm::
+- Checkmate detection (working on)
+
+Stuff that will probably never be implemented:
+- Pawn promotion - pawns are still useful after they reach the back rank because of the tiles, and because of the tiles it's easier to get them to the back rank
+
+If you like this, please star on github!