Spelling fix, comments explaining script.

tools/install_dependencies.sh

 #!/bin/sh
 # Copyright 2014 Google Inc.
 #
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# install_dependencies.sh will install system-specific Skia
+# dependencies using your system's package manager.  If your system is
+# not supported, add logic here to support it.
+
 if command -v lsb_release > /dev/null ; then
     case $(lsb_release -i -s) in
         Ubuntu)
             sudo apt-get install \
                 build-essential \
                 libfreetype6-dev \
                 libfontconfig-dev \
                 libpng12-dev \
                 libgif-dev \
                 libqt4-dev \
                 clang
             if [ $(lsb_release -r -s) = '14.04' ] ; then
                 sudo apt-get install \
                     ninja-build
             fi
             exit
             ;;
     esac
 fi
 
 echo 'unknown system'
 exit 1