Readme updates

[greena13]

This pull request includes, imho, a bunch of improvements to the project's readme.

I gathered this information by:

• Using and experimenting with the package (master - not the latest version on npm)
• Reading through the test suite
• Reading through the source code (and the source code of some of the dependencies)

I believe it's all correct, but I can't guarantee that there are no misunderstandings or errors contained within.

Open to suggestions for changes of formatting, rewording, corrections or anything, really.

What's changed

Document structure

• Introduced some more logical structer and recognisable section headings for users skim reading the documentation
• Applied markdown more consistently for headings
• Made it more apparent that React Shortcuts is based on Atom's keymaps (a name people will recognise and trust)
• May have reordered some sections for document flow - I can't remember
• Fixed typos and formatting issues
• Added a lot of local links, so relevant information and elaborations can be easily followed without duplication

How it works section

• Explained the relationship between <Shortcuts> and the contents of the keymap
• Explained the order used for matching keyboard shortcuts (particularly relevant for nested <Shortcuts>)
• Explained the default behaviour and what happens when you use the stopPropagation and global properties in terms of triggering and propagation events

Integrating with existing onKeyDown, onKeyUp and onKeyPress handlers

• Explained the default behaviour of React shortcuts with regular React key listeners and how to change it using the isolate prop

Handler function section

• Explained when a shortcut handler is and is not called, linking to relevant sections for ellaboration

<Shortcuts>'s props

• Moved into a table for easier reading
• Added a default value for all props (where applicable)
• Improved the description of each, linking to relevant ellaborations

Managing focus in the browser

• Explained how browser focus works and the role that tabindex plays - this section is somewhat like an appendix but ended up being pre-requisite knowledge for understanding how to use this package for me, so I think it has a place in the readme
• Linked to relevant sections and behaviour of React Shortcuts

Troubleshooting section

• Added troubleshooting section for problems I encountered while using this project (and those I have seen others experience)
• Now discoverable by browsing or using the browser's search function

Let me know what you think. :)