Add a top-level README

README.md

+ai3/go-common
+===
+
+Common code for ai3 services and tools.
+
+A quick overview of the contents:
+
+* [client](clientutil/) and [server](serverutil/) HTTP-based
+  "RPC" implementation, just JSON POST requests but with retries,
+  backoff, timeouts, tracing, etc.
+
+* [server implementation of a line-based protocol over a UNIX socket](unix/)
+
+* a [LDAP connection pool](ldap/)
+
+* a [password hashing library](pwhash/) that uses fancy advanced
+  crypto by default but is also backwards compatible with old
+  libc crypto.
+
+* utilities to [manage encryption keys](userenckey/), themselves
+  encrypted with a password and a KDF.
+