docs(readme): add table of contents

[macklinu]

This PR aims to clean up the README a little bit and add a table of contents. I see this as a first step to resolving #61.

I generated the table of contents by running npx doctoc README.md --notitle --maxlevel=3. I wanted to set up lint-staged in this PR, but I had trouble installing it using Node 4 and npm 2 in my local development environment. (I'll open an issue to discuss using Node 8 and npm 5 as the preferred development environment, but continue running tests on Node 4 to continue supporting that version of Node with this ESLint plugin.)

The primary change is that all rules are documented below the Rules section with the following convention:

### `rule-name`

This allows for consistent documentation URLs, which now look like https://github.com/xjamundx/eslint-plugin-promise#rule-name.

Once this is merged, I will open a follow up PR to add the meta.docs.url field to all rules, which should close #61. (ESLint did a similar thing in their codebase in eslint/eslint#9788)