Docs: Added workaround for testing on RedHat OSs

Since Scancode binaries are built on Debian based machines, and the
prebuilt dependencies are named differently on RedHat based machines
py.test will not be able to find the libbz2.so library on those
machines.
See https://github.com/nexB/scancode-toolkit/issues/443

Added a note about how to work around this issue by symbolically
linking the existing libbz2.so on the filesystem to the expected
name.

This should be removed once
aboutcode-org#469 is solved

Signed-off-by: Nisha K <nishak@vmware.com>

CONTRIBUTING.rst

@@ -119,6 +119,19 @@ To set up ScanCode for local development:
 
        py.test -n6
 
+   If you are running this on a RedHat based OS you may come across this
+   failure::
+   
+       OSError: libbz2.so.1.0: cannot open shared object file: No such file or directory
+ 
+   Try creating a symbolic link to libbz2.so.1.0 to solve this issue::
+
+       locate libbz2.so.1.0
+       cd <resulting libbz2.so directory>
+       sudo ln -s <your version of libbz2.so> libbz2.so.1.0
+      
+   See https://github.com/nexB/scancode-toolkit/issues/443 for more information.
+
 7. Commit your changes and push your branch to your GitHub fork::
 
     git add .