Added requested documentation (#212)

Added usage documentation for -extra-plugins argument, with usage examples.

README.md

@@ -34,6 +34,7 @@ Options:
    -always-overwrite     : Copy files even if the target file exists
    -qmake=<path>         : The qmake executable to use
    -no-translations      : Skip deployment of translations
+   -extra-plugins=<list> : List of extra plugins which should be deployed, separated by comma
 
 linuxdeployqt takes an application as input and makes it
 self-contained by copying in the Qt libraries and plugins that
@@ -80,6 +81,18 @@ find $HOME/build-*-*_Qt_* \( -name "moc_*" -or -name "*.o" -or -name "qrc_*" -or
 
 Alternatively, you could use `$DESTDIR`.
 
+#### Adding extra Qt plugins 
+
+If you want aditional plugins which the tool doesn't deploy, for a variety of reasons, you can use the -extra-plugins argument and include a list of plugins separated by a comma.  
+The plugins deployed are from the Qt installation pointed out by `qmake -v`.  
+You can deploy entire plugin directories, a specific directory or a mix of both.
+
+Usage examples: 
+
+1. `-extra-plugins=sqldrivers/libqmsql.so,iconengines/libqsvgicon.so`  
+2. `-extra-plugins=sqldrivers,iconengines/libqsvgicon.so`  
+3. `-extra-plugins=sqldrivers,iconengines,mediaservice,gamepads`
+
 ## Using linuxdeployqt with Travis CI
 
 A common use case for `linuxdeployqt` is to use it on Travis CI after the `make` command. The following example illustrates how to use `linuxdeployqt` with Travis CI. Create a `.travis.yml` file similar to this one (be sure to customize it, e.g., change `APPNAME` to the name of your application as it is spelled in the `Name=` entry of the `.desktop` file):