Improvements to the data sharing motivation

1. Changed structure to highlight advantages to both the
scientific community and the data sharer in the second paragraph.

2. Changed the third paragraph to emphasize the the issue is with
incentives, not researchers, and to highlight that the incentives
have now changed to make sharing worthwhile.

3. Added additional citations
Merge pull request #121 from ethanwhite/improve-motivation

Improvements to the data sharing motivation (Fixes #92)