DOC: Provide More Guidance for Contributors

Explain the process to a new contributor in more detail.

Updated the structure and formatting, added some useful git commands
and a link to ITK Bar Camp pages.

Added a couple more commands and changes suggested by Matt McCormick.

Change-Id: I2847d53bc98ee891bf14a2101eed6e369c13c2ca
Merge "DOC: Provide More Guidance for Contributors"