doc: Update list of dependencies and add pysispm one

Change-Id: I05cc63bfd627bfec83b50fe89fe8004d130384e6
diff --git a/doc/README.txt b/doc/README.txt
index c00cd3e..3b4d202 100644
--- a/doc/README.txt
+++ b/doc/README.txt
@@ -23,9 +23,11 @@
   python3-mako \
   python3-gi \
   ofono \
-  python3-pip
+  python3-pip \
+  python3-usb
   pip3 install git+git://github.com/podshumok/python-smpplib.git
   pip3 install pydbus
+  pip3 install pysispm
 
 To build ofono:
   apt-get install libglib2.0-dev \