06-merging-data: added explanatory text

Replaced the stub/outline text from the previous commit (50b17d36) with
actual text explaining how various types of joins available in panadas.
06-merging-data: stub for DataFrame concat

Stubbed out section for explaning DataFrame concatenation using pandas;
i.e., pandas-equivalents for R's cbind() and rbind() functions.
Merge pull request #6 from chenghlee/mozsprint-atx

Text for joining using pandas