README: reference 'fast' extra instead of 'full'

'full' needs additional OS packages (for hw) that are not mentioned

README.rst

@@ -41,11 +41,11 @@ directory. To run Electrum from its root directory, just do::
 You can also install Electrum on your system, by running this command::
 
     sudo apt-get install python3-setuptools
-    pip3 install .[full]
+    pip3 install .[fast]
 
 This will download and install the Python dependencies used by
 Electrum, instead of using the 'packages' directory.
-The 'full' extra contains some optional dependencies that we think
+The 'fast' extra contains some optional dependencies that we think
 are often useful but they are not strictly needed.
 
 If you cloned the git repository, you need to compile extra files
@@ -64,9 +64,9 @@ Check out the code from GitHub::
 
 Run install (this should install dependencies)::
 
-    pip3 install .[full]
+    pip3 install .[fast]
 
-Render the SVG icons to PNGs::
+Render the SVG icons to PNGs (optional)::
 
     for i in lock unlock confirmed status_lagging status_disconnected status_connected_proxy status_connected status_waiting preferences; do convert -background none icons/$i.svg icons/$i.png; done