[readme] Document requirement for go 1.11+

Change-Id: If001fc048d595b806d36b5259459caa44ba7e8fd

README.md

diff --git a/README.md b/README.md
index 2928909..c92ea8b 100644
--- a/README.md
+++ b/README.md
@@ -22,7 +22,7 @@
   makes compilation significantly faster; if both are present, Ninja will be
   used. On Windows, Ninja is required.
 - A C compiler
-- Go
+- Go 1.11 or later
 - To build the x86 and x86_64 assembly, your assembler must support AVX2
   instructions and `MOVBE`. If using GNU binutils, you must have 2.22 or later.