Improve Readme

[jkowens]

The readme could use a makeover 💅

Adding details for the following issues will be a good start. Pulling in some key information from the wiki would be a good idea too.

• Returns information should be in the readme #174
• Add Examples to Readme #229
• Use with Apartment gem leads to wrong sequence being used on postgresql #233
• document import! #318
• Gemfile order with composite_primary_keys gem #396
• Document lack of uniqueness validation #430
• before_save, before_create, before_update #442
• How to require activerecord-import? #474
• Document .bulk_import method #505
• Importing an array of hashes ignores columns not in the first hash #507

[tibbon]

Hmm, I might take a swing at this when I get a chance.

[jkowens]

@tibbon that would be fantastic!!

[dillonwelch]

#556 checked off #474

[dillonwelch]

As these get updated, how do you feel about replacing the content of the wikis with links to the appropriate README sections?

[dillonwelch]

Also, for #396 the OC ended up closing out the issue because it went stale and he wasn't able to provide a demo since he doesn't work on that project anymore

[jkowens]

Regarding the the wiki, that sounds good. Add links to the readme and eventually we will probably just delete the entries.