Fix README rendering on PyPI
Fix README rendering on PyPI

The rST parser used on PyPI rejects anything with warnings, which is why the rendering is broken for the current version: https://pypi.org/project/filelock/3.0.9/

This can be checked with:

```
$ pip install docutils pygments
$ python setup.py check --metadata --restructuredtext
running check
warning: check: Duplicate implicit target name: "similar libraries".

warning: check: Duplicate implicit target name: "installation".

warning: check: Duplicate implicit target name: "documentation".

warning: check: Duplicate implicit target name: "contributions".

warning: check: Duplicate implicit target name: "license".
```

I fixed the warnings by removing the 'ToC' at the top. The README isn't so long so I don't think this is a huge loss. After this change, the check passes:

```
$ python setup.py check --metadata --restructuredtext
running check
```

benediktschmitt/py-filelock,tox-dev/py-filelock