Docs: Add better usage instructions to docs

[dbrgn]

In #108 the argument parsing was switched from docopt to clap. This resulted in the removal of the examples:

Examples:

    $ tldr tar
    $ tldr --list

To control the cache:

    $ tldr --update
    $ tldr --clear-cache

To render a local file (for testing):

    $ tldr --render /path/to/file.md

The usage docs should probably be updated to include some examples like this. In general, we should explain to the user how tealdeer can be used. (This can be done through examples, but other means are possible as well.)