Slightly more verbose/helpful docs.

[beausmith]

Update install and usage docs to be slightly more verbose, obvious, and helpful.

[beausmith]

Very cool project!

It took me too long to figure out how to use it for two reasons:

• In overlooked the "Install asciinema" comment when skimming the readme thinking the comment wouldn't contain anything critical… so I separated the two steps into an ordered list. Now the URL to https://asciinema.org/docs/installation is hyperlinked, too!
• I couldn't figure out where the "113643" came from in the usage example until I clicked the link, so I placed the url in the rendered page.

[marionebl]

Thanks!