clarify recommended boost versions - closes #1001

INSTALL.md

@@ -36,7 +36,11 @@ Build dependencies are:
 
 Mapnik Core depends on:
 
- * Boost >= 1.42.x (>= 1.45.x if using clang++) with these libraries:
+ * Boost
+    - >= 1.47 is recommended
+    - >= 1.45.x is required if compiling with clang++
+    - >= 1.42.x works on most systems and most compilers
+    - These libraries are required:
       - filesystem
       - system
       - thread (if mapnik threadsafe support is required, default on)