INSTALL: document USE_SSE

Summary:
Fixes #1790.

adamretter siying
Closes https://github.com/facebook/rocksdb/pull/1996

Differential Revision: D4730449

Pulled By: IslamAbdelRahman

fbshipit-source-id: 19944fa

INSTALL.md

@@ -22,7 +22,8 @@ use binaries compiled by `make all` in production.
 
 * By default the binary we produce is optimized for the platform you're compiling on
 (-march=native or the equivalent). If you want to build a portable binary, add 'PORTABLE=1' before
-your make commands, like this: `PORTABLE=1 make static_lib`
+your make commands, like this: `PORTABLE=1 make static_lib`. If you want to build a binary that
+makes use of SSE4, add 'USE_SSE=1' before your make commands, like this: `USE_SSE=1 make static_lib`.
 
 ## Dependencies