Update README.rst

Just including the Salesforce API link to objects that can be created. Adding some extra clarification.
Update README links
Changes made to links
Merge pull request #46 from ajoys/patch-1

Update README.rst
Add `SalesforceError` as base API exception
Authenitcation error derive form `SalesforceError`
Move `SalesforceError` to utils to avoid mutual dependecy
Merge pull request #49 from nbessi/common_exception

Add `SalesforceError` as base API exception
Merge pull request #1 from neworganizing/master

pull from neworganizing:master