Added initial version of the README to explain the library

Author: amcewen

README

@@ -0,0 +1,9 @@
+HttpClient is a library to make it easier to interact with web servers from Arduino.
+
+Dependencies:
+- Requires the new Ethernet library API (with DHCP and DNS) which is in Arduino 1.0.
+- Requires the b64 base64 encoding library, available from https://github.com/amcewen/b64
+
+In normal usage, handles the outgoing request and Host header.  The returned status code is
+parsed for you, as is the Content-Length header (if present).
+