[doc] Document C++14 in C++ Style Guide

This matches the C++ version mandated in the "System Requirements" document.

Signed-off-by: Sam Elliott <selliott@lowrisc.org>
diff --git a/doc/rm/c_cpp_coding_style.md b/doc/rm/c_cpp_coding_style.md
index ece50a2..e650570 100644
--- a/doc/rm/c_cpp_coding_style.md
+++ b/doc/rm/c_cpp_coding_style.md
@@ -130,6 +130,12 @@
 int create_rainbow(int pots_of_gold, int unicorns);
 ```
 
+## C++ Style Guide
+
+### C++ Version
+
+C++ code should target C++14.
+
 
 ## C Style Guide