CONTRIBUTING.md: various improvements #2594
CONTRIBUTING.md: various improvements #2594

Changes from top to bottom of this doc:

- Say it's ok to email support privately about ideas.
- Standardize on calling GitHub issue the "issue tracker".
- Link to http://chat.dataverse.org rather than freenode web interface
  and explicitly mention IRC logs.
- When writing an issue, include steps to reproduce, version number, and
  screenshots.
- Explain that many more attachment formats are now accepted.
- Link to new documentation section of dev guide.
- Link to doc folder in "develop" rather than "master".

jacksonokuhn/dataverse,JayanthyChengan/dataverse,jacksonokuhn/dataverse,jacksonokuhn/dataverse,jacksonokuhn/dataverse,JayanthyChengan/dataverse,JayanthyChengan/dataverse,JayanthyChengan/dataverse,JayanthyChengan/dataverse,JayanthyChengan/dataverse,JayanthyChengan/dataverse,JayanthyChengan/dataverse,jacksonokuhn/dataverse,jacksonokuhn/dataverse,jacksonokuhn/dataverse,jacksonokuhn/dataverse