gollvm: Updating README.md, to elevate the min. required version of Clang compiler

Too many issues with Clang 9 - so let's assume Clang 10+

Change-Id: Icf1a74148878b07fd941e5d525ac7e0c7f6bfdaf
Reviewed-on: https://go-review.googlesource.com/c/gollvm/+/275473
Reviewed-by: Than McIntosh <thanm@google.com>
Trust: Ian Lance Taylor <iant@golang.org>

README.md

diff --git a/README.md b/README.md
index 65e7369..0c563c9 100644
--- a/README.md
+++ b/README.md
@@ -66,7 +66,7 @@
 
 ## Building gollvm with cmake and ninja <a name="cmakeninja"></a>
 
-You'll need to have an up-to-date copy of cmake on your system (3.6 or later vintage) to build Gollvm, as well as a C/C++ compiler (V5.0 or later for Clang, or V6.0 or later of GCC), and a working copy of 'm4'.
+You'll need to have an up-to-date copy of cmake on your system (3.6 or later vintage) to build Gollvm, as well as a C/C++ compiler (V10.0 or later for Clang, or V6.0 or later of GCC), and a working copy of 'm4'.
 
 Create a build directory (separate from the source tree) and run 'cmake' within the build area to set up for the build. Assuming that 'workarea' is the directory created as above: