Update documentation.

Author: mackron

c89atomic.h

@@ -7,10 +7,10 @@ David Reid - mackron@gmail.com
 /*
 Introduction
 ============
-This library aims to implement an equivalent to the C11 atomics library. It's intended to be used as a way to
-enable the use of atomics in a mostly consistent manner to modern C, while still enabling compatibility with
-older compilers. This is *not* a drop-in replacement for C11 atomics, but is very similar. Only limited testing
-has been done so use at your own risk. I'm happy to accept feedback and pull requests with bug fixes.
+This is a mostly C89 compatible atomics library. It's intended to be used as a way to enable the use of atomics
+in a mostly consistent manner to modern C, while still enabling compatibility with older compilers. This is
+*not* a drop-in replacement for C11 atomics, but is very similar. Only limited testing has been done so use at
+your own risk. I'm happy to accept feedback and pull requests with bug fixes.
 
 The following compilers are supported:
 
@@ -22,6 +22,7 @@ The following compilers are supported:
   - OpenWatcom (Tested with 2.0)
   - Digital Mars
   - Borland C++ (Tested with 5.02)
+  - chibicc
 
 New compilers will use intrinsics. GCC-likes, such as GCC and Clang, will use `__atomic_*` intrinsics through a
 pre-processor define and should have no overhead. This uses `__GNUC__` to detect GCC-likes.
@@ -32,6 +33,10 @@ of these two paths. Note that only 32- and 64-bit x86 is supported for inlined a
 tested the inlined assembly paths. It passes basics tests, but things like memory ordering may have some issues.
 Advice welcome on how to improve this.
 
+For practicality this library supports 64-bit atomics and is therefore not strictly C89 compatible since C89
+does not technically support 64-bit integers (these are compiler extensions). However, it should still work on
+old compilers. In addition, this library currently assumes `int` is 32 bits.
+
 
 Differences With C11
 --------------------