Revise English in the intro in the documentation

Hi,

Thanks for sharing this!

Though I'm not a native English writer, 
I have some suggestions to to make your documents a bit more concise.

You should:

- at first write a what-it-is in the "Cascade is a *blah-blah-blah* which does *some cool things for us programmers*" form
- not just repeat the same explanation in different terms, not to confuse the readers
- differentiate `unified` from `integrated`
- use more concrete words than "storage" because it may be taken as "computer component/physical device to retain data", but not relational/NoSQL databases, files, caching systems that you meant.

Forgive me if I'm being rude.
I hope this helps!

Regards,

Yusuke
Fix typos in the intro in the doc

Exactly as the title says :)
Merge remote branch 'remotes/mumoshu/patch-1' into develop
Merge remote branch 'remotes/mumoshu/patch-2' into develop

Conflicts:
	doc/intro.rst