Added more detail to the README

Author: PeterMartini

README.md

@@ -2,3 +2,14 @@ docker-perl
 ===========
 
 Dockerfiles for Perl5
+
+The structure of this repo is to use the full version ID of each Perl version,
+plus a comma separate list of extensions.  Every directory is expected to have
+at least the bit specification (32bit or 64bit), and at the moment the only
+other extension is threaded.
+
+There are currently no 32bit extensions as Docker does not (yet?) support 32-bit
+builds.
+
+The 64bit builds specify use64bitall despite this being largely redundant
+(Configure would properly detect this) to make the desired bit size explicit.