Fix for issue #521 (add new features about drag&drop between libraries)

[GNAQ]

Describe the changes you have made here: what, why, ...
This is a pull request of us(students of HIT),and we add one line at the bottom of readme.md for this PR.
We attempted to make some contributions to jabref and this is a test for later.

Link issues that are fixed:
Merge some (or all) of the "new entry" related to #521
Fixed:#521

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.