Update contribution guide and documentation

[tanujjain]

In the contribution guide, add the following:

1. Use of single quotes instead of double
2. import order and groups as specified python org (https://www.python.org/dev/peps/pep-0008/#imports). Also mention that regardless of import form, all imports from the same dependency are to be put in consequent lines.

In the documentation, update the find_duplicates documentation to reflect that encoding_map parameter of find_duplicates expects the value in the dictionary to be of shape (1024,). This led to a confusion as reported in #114