Add a README

README.md

+webdav-server-go
+===
+
+The WebDAV server component, meant to run in the
+[users-dav](https://git.autistici.org/ai3/docker/apache2-users-dav)
+container. Authenticates users against the [webdav-auth authentication
+server](https://git.autistici.org/ai3/tools/webdav-server).
+
+It is written in Go, and runs as a FastCGI server.
+
+