DOC: need SQL -> pandas tips
of course #4164 makes this syntax easier anyhow

Here's an example of the types of things we need (obviously not all apply here)
http://www.pytables.org/moin/HintsForSQLUsers

we have this in the docs for join/merge, but maybe useful for:
- groupby
- boolean indexing to replace select

e.g.

```
select * from mytables where A='bar' and B>to_date('20130101')
```

replaced by:

`df[(df.A=='bar') & (df.B>Timestamp('20130101'))`

```
select count(*) from mytables
```

replaced by

`len(df)`

etc
