README: add quotes in pip command for zsh users

Fix #347

Author: rossengeorgiev

README.rst

@@ -36,7 +36,7 @@ Install latest release version from PYPI:
 .. code:: bash
 
     # with SteamClient dependecies
-    pip install -U steam[client]
+    pip install -U 'steam[client]'
 
     # without (only when using parts that do no rely on gevent, and protobufs)
     pip install -U steam
@@ -46,12 +46,12 @@ Installing directly from ``github`` repository:
 .. code:: bash
 
     # cutting edge from master
-    pip install git+https://github.com/ValvePython/steam#egg=steam
+    pip install 'git+https://github.com/ValvePython/steam#egg=steam'
 
     # specific version tag (e.g. v1.0.0)
-    pip install git+https://github.com/ValvePython/[email protected]#egg=steam[client]
+    pip install 'git+https://github.com/ValvePython/[email protected]#egg=steam[client]'
     # without SteamClient extras
-    pip install git+https://github.com/ValvePython/[email protected]#egg=steam
+    pip install 'git+https://github.com/ValvePython/[email protected]#egg=steam'
 
 Vagrant
 -------