Mention FreeBSD support in the README (#737)

diff --git a/README.md b/README.md
index b3ae83f..b7280b9 100644
--- a/README.md
+++ b/README.md
@@ -54,6 +54,7 @@
 * Linux
 * OS X
 * Windows
+* FreeBSD
 * NetBSD
 * Android
 * iOS