State explicitly in README.md that QUICHE is little-endian only.

PiperOrigin-RevId: 337375886
Change-Id: Icace62dfaf7f5901f709c2ee6b24eae3e56eed07

README.md

diff --git a/README.md b/README.md
index 51b6acc..53707ec 100644
--- a/README.md
+++ b/README.md
@@ -2,4 +2,4 @@
 
 QUICHE (QUIC, Http/2, Etc) is Google's implementation of QUIC and related
 protocols.  It powers Chromium as well as Google's QUIC servers and some other
-projects.
+projects.  QUICHE is only supported on little-endian platforms.