readme: add info about dependencies

So far, the README.md file does not mention much about the
dependencies. This commt adds at lest the most important ones

Change-Id: I912bbf787e1408100183cade2113bf7617b86ffa

README.md

@@ -28,6 +28,15 @@ You can clone from the official libosmocore.git repository using
 
 There is a cgit interface at <http://git.osmocom.org/pysim/>
 
+
+Dependencies
+------------
+pysim requires pyscard and pytlv. Both can be installed using python-pip:
+
+pip install pytlv
+pip install pyscard
+
+
 Mailing List
 ------------