JBIDE-17404 Adding instructions for RHEL7 and CordovaSim

I have following installed on RHEL7 machine:

```
[root@kpiwko-x220 kpiwko]# rpm -qa '*webkit*' | xargs yum info
Loaded plugins: langpacks, product-id, subscription-manager
Installed Packages
Name        : libwebkit2gtk
Arch        : x86_64
Version     : 2.0.4
Release     : 6.el7_0.1
Size        : 66 M
Repo        : installed
From repo   : rhel-7-workstation-optional-rpms
Summary     : The libwebkit2gtk library
URL         : http://www.webkitgtk.org/
License     : LGPLv2+ and BSD
Description : The libwebkit2gtk package contains the libwebkit2gtk library
            : that is part of webkitgtk3.

Name        : webkitgtk
Arch        : x86_64
Version     : 2.2.4
Release     : 1.el7
Size        : 42 M
Repo        : installed
From repo   : epel
Summary     : GTK+ Web content engine library
URL         : http://www.webkitgtk.org/
License     : LGPLv2+ and BSD
Description : WebKitGTK+ is the port of the portable web rendering engine WebKit to the
            : GTK+ platform.

Name        : webkitgtk3
Arch        : x86_64
Version     : 2.0.4
Release     : 6.el7_0.1
Size        : 44 M
Repo        : installed
From repo   : rhel-7-workstation-rpms
Summary     : GTK+ Web content engine library
URL         : http://www.webkitgtk.org/
License     : LGPLv2+ and BSD
Description : WebKitGTK+ is the port of the portable web rendering engine WebKit to the
            : GTK+ platform.
            : 
            : This package contains WebKitGTK+ for GTK+ 3.

Name        : webkitgtk3-devel
Arch        : x86_64
Version     : 2.0.4
Release     : 6.el7_0.1
Size        : 5.0 M
Repo        : installed
From repo   : rhel-7-workstation-optional-rpms
Summary     : Development files for webkitgtk3
URL         : http://www.webkitgtk.org/
License     : LGPLv2+ and BSD
Description : The webkitgtk3-devel package contains libraries, build data, and header
            : files for developing applications that use webkitgtk3.
```

Let's remove the one coming from EPEL:

```
yum erase webkitgtk
```

Let's run JBDS 8.0.0.CR1 and create _contacts-mobile-basic_:

Run As/ Run With CordovaSim fails with:

```
BrowserSim has failed to start with the following error:
"To use CordovaSim please install webkitgtk 1.2.0 or higher or run CordovaSim with Oracle Java 7u7 and higher."
Also see <a href="http://tools.jboss.org/documentation/faq/browsersim.html">BrowserSim FAQ</a>.
Stack trace:
org.eclipse.swt.SWTError: To use CordovaSim please install webkitgtk 1.2.0 or higher or run CordovaSim with Oracle Java 7u7 and higher.
	at org.jboss.tools.vpe.cordovasim.CordovaSimRunner.startCordovaSim(CordovaSimRunner.java:109)
	at org.jboss.tools.vpe.cordovasim.CordovaSimRunner.main(CordovaSimRunner.java:91)
```

Note I'm running on Oracle JDK 7 - 1.7.0.65-1jpp.1.el7.x86_64.

If switched to GTK3, CordovaSim starts fine. Hence my original suggestion making GTK3 default.

After reverting back to GTK2, installing EPEL package and restarting JBDS I can run on GTK2