Removed adduser $USER fuse instructions for all but Debian/Ubuntu.

The `adduser` syntax is different on the other distributions (the
command doesn't even exist on sles-12). More importantly, installing
fuse doesn't seem to even create a group called `fuse`. When I Google
for context, the only instructions saying to add the user to the group
seem to all be related to Ubuntu and Debian, so this appears to only be
relevant there.

For #95.

docs/installing.md

@@ -11,8 +11,8 @@ It may or may not work correctly with other operating systems and older versions
 
 If you are running Linux on a 64-bit x86 machine and are happy to install
 pre-built binaries (i.e. you don't want to build from source), you need only
-ensure fuse is installed and configured, then download and extract the latest
-release. The instructions slightly vary by distribution.
+ensure fuse is installed, then download and extract the latest release. The
+instructions slightly vary by distribution.
 
 ## Debian and Ubuntu
 
@@ -42,23 +42,15 @@ you may need to run the workaround above repeatedly.
 ## CentOS and Red Hat
 
     sudo yum install wget fuse
-    sudo adduser $USER fuse
     wget https://github.com/GoogleCloudPlatform/gcsfuse/releases/download/v0.5.0/gcsfuse_v0.5.0_linux_amd64.tar.gz
     sudo tar -C /usr/local/bin -zxf gcsfuse_v0.5.0_linux_amd64.tar.gz
 
-You may need to log out and then log back in to make sure that the change to
-the `fuse` group takes effect.
-
 ## SUSE
 
     sudo zypper install wget fuse
-    sudo adduser $USER fuse
     wget https://github.com/GoogleCloudPlatform/gcsfuse/releases/download/v0.5.0/gcsfuse_v0.5.0_linux_amd64.tar.gz
     sudo tar -C /usr/local/bin -zxf gcsfuse_v0.5.0_linux_amd64.tar.gz
 
-You may need to log out and then log back in to make sure that the change to
-the `fuse` group takes effect.
-
 
 # OS X