Added contributor and updated README to add AIX.

CONTRIBUTORS.txt

@@ -91,3 +91,6 @@ Patch contributors:
   William Orr <will@worrbase.com>
     * Fixed detection of sched_yield on Solaris.
     * Added atomicops for Solaris
+  Andrew Paprocki <andrew@ishiboo.com>
+    * Fixed minor IBM xlC compiler build issues
+    * Added atomicops for AIX (POWER)

README.md

@@ -127,6 +127,14 @@ For advanced usage information on configure and make, see INSTALL.txt.
 
   Also, you will need to use gmake instead of make.
 
+**Note for AIX users**
+
+  Compile using the IBM xlC C++ compiler as follows:
+
+    ./configure CXX=xlC
+
+  Also, you will need to use GNU `make` (`gmake`) instead of AIX `make`.
+
 C++ Installation - Windows
 --------------------------