Create some initial description/instructions for this directory.

xtrn/3rdp-install/README.md

+# Synchronet 3rd Party External Program (door) Install Instructions
+
+The `.ini` files in this directory are in the Synchronet `install-xtrn.ini`
+format with base filenames reflecting the door they are intended to help
+install into a Synchronet system.
+
+The additional keys expected are:
+~~~
+exe = name of the executable file
+md5 = comma-separated list of MD5-sums for supported executable file versions
+~~~
+MD5-sums are presented in lowercase hexadecimal (e.g. same format returned by
+*nix `md5sum` utility).