diff --git a/README.rst b/README.rst
index 7320d456..aab76fd4 100644
--- a/README.rst
+++ b/README.rst
@@ -145,8 +145,10 @@ minimizes the risk of private key leakage when serious bug like
 Heartbleed is exploited.  The neverbleed is disabled by default.  To
 enable it, use ``--with-neverbleed`` configure option.
 
-In order to compile the source code, gcc >= 6.0 or clang >= 6.0 is
-required.  C++ source code requires C++14 language features.
+Compiling libnghttp2 C source code requires a C99 compiler.  gcc 4.8
+is known to be adequate.  In order to compile the C++ source code, gcc
+>= 6.0 or clang >= 6.0 is required.  C++ source code requires C++14
+language features.
 
 .. note::