Use readme file as long description in setup file

[ghost]

This change uses the README.md contents as long_description in setuptools.setup call so that the project description is available on PyPI. Currently, the project on PyPI shows no description at all: https://pypi.org/project/lambda-uploader.

The redundant readme file is also removed. The .rst version was removed since it was missing one section that was in the .md version (the subscription configuration section). Because of this, it was assumed that the markdown version was the preferred one. It will be much easier to maintain only one readme file.

It is important to note that in order to correctly display the markdown formatted description on PyPI, setuptools>=38.6.0 must be used to create the builds and twine>=1.11.0 must be used to upload them. You can see here for more information.