Update README.md file, use DB as storage

[shqear93]

@alexreisner I would like to hear your input about this, this is very optional addtition to the reademe file that might be helpful for some users

[alexreisner]

@shqear93 I like this. Two suggestions:

• This should go in the "Caching" section, right above the last line, which is in italics ("Before you implement caching...").
• I'm trying to reduce the amount of text in the README. Can this be a single line?: "In addition to conventional cache stores like Redis, it's possible to keep your cache in the database using ActiveRecord. For example see this gist."