Combine models with retain_original=False, and docs

[thallada]

Thanks for making this library, it's really great and easy to use. I was trying to load a large corpus file-by-file and noticed an error when trying to use markovify.combine on models with retain_original=False:

AttributeError: 'Text' object has no attribute 'parsed_sentences'

I think this change fixes the issue.

And, I documented how to create models file-by-file and combine them into one model in the README. I was wondering how to do that when I first started using markovify.

Also, kind of unrelated, but I added a tip about using spaCy to the README too.

[coveralls]

Coverage remained the same at 100.0% when pulling 9a6f14d on thallada:combine-retain-original-false into 4d62bfb on jsvine:master.

[jsvine]

Thank you, Tyler! Great catch and great fix. Much appreciated.

[jsvine]

Merged, and now available in 0.6.3.

(And nice tip about spaCy.)