Suggestion to put on README file

[rodrigofariasm]

Hi, thanks for the lib, it is very useful.
I suggest that you put on the README file that is just the:
$lein datomic initialize command that cleans the DB
you can stop the database and run:
$lein datomic start &
command and use the already existent DB once it was already initialized

Sorry if this is deductible, but always is better to explicit.