Added a table of contents

Added table of contents to README.md generated with [DocToc](http://doctoc.herokuapp.com/)

I pointed doctoc at the README.md on the tip of master via the following URL, and pressed the button:
https://github.com/protobuf-c/protobuf-c/blob/master/README.md

I then just paster the full generated TOC markdown blob, added a TAC section header, and moved/tweaked the DocToc shout-out the end of the blob. Feel free to remove the shoutout if desired.

Original shout-out:

```markdown
**Table of Contents**  *generated with [DocToc](http://doctoc.herokuapp.com/)*
```

Tweaked shout-out:

```markdown
```

Since the TOC is now in its own 'section', generating an updated TOC will have an entry for the TOC section which can just be removed manually.