Update README.md to include the seach for the _id from string and chaining the where statements

Id search
<code>
LMongo::collection('users')
->where('_id',new MongoId("$_id"))
</code>

Chaining where statements
<code>
LMongo::collection('users')
->where('email', $email)
->andWhereNe('first_name', $first_name)
->get()
</code>