Adding a file for Perl (5.20.0 and 5.18.2)

The referenced repo has 4 directories right now, 2 for
each version of Perl.  One has command line options
just for 64-bit compilation (which would actually be turned
on by default anyway, but I wanted to be explicit about it),
and the other will also enable threads.  Threads are generally
discouraged, so they specifically need to be opted into.

library/perl

@@ -0,0 +1,21 @@
+# Maintainer: Peter Martini <[email protected]> (@PeterMartini)
+
+latest: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit
+
+5: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit
+
+5.20: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit
+5.20.0: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit
+
+5.18: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.018.002-64bit
+5.18.2: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.018.002-64bit
+
+latest-threaded: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit,threaded
+
+5-threaded: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit,threaded
+
+5.20-threaded: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit,threaded
+5.20.0-threaded: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.020.000-64bit,threaded
+
+5.18-threaded: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.018.002-64bit,threaded
+5.18.2-threaded: git://github.com/petermartini/docker-perl@a2e267efb9943a7695dadfe7a152c4c1620710ea 5.018.002-64bit,threaded