Modified application/migration guide for version 0.5.1 (#290).
Merged pull request (#286, #290).

Delete a wrong sentence.

This sentence is not suitable for an explanation of Extract operator.
I guess that this paragraph was copied from an explanation of CoGroup
operator.
Fixed release note bugs on version 0.5.0 (#290).
Fixed product/road-map document bugs (#290).
Fixed document typo (#290).
Fixed document layout (#290).
Add documetns (#287, #290)

For managing application dependency libraries per project.
Modified documents for version 0.5.1 (#290).
Revised documents for dataflow tracing (#290, #291).
Added changelogs for version 0.5.1 (#290).
Added release notes for version 0.5.1 (#290).
Merge remote-tracking branch 'private/issue/290' into 0.5-develop
Merge remote-tracking branch 'central/0.5-develop' into 0.5-release
Bumped version number to 0.5.1