You can keep up with the latest developments in MySQL software in a number of ways, including various blogs and other channels. However, for the most correct (if somewhat dry and factual) information, you can go directly to the source.
Major Releases
For every major release, the MySQL docs team creates and maintains a "nutshell" page containing the significant changes in that release. For the current GA release (whatever that is) you'll find it at this location:
https://dev.mysql.com/doc/mysql/en/mysql-nutshell.html
At the moment, this redirects to the summary notes for MySQL 5.6. The notes for MySQL 5.7 are also available at that website, at the URL http://dev.mysql.com/doc/refman/5.7/en/mysql-nutshell.html, and when eventually that version goes GA, it will become the currently linked notes from the URL shown above.
Incremental Releases
For more detail on each incremental release, you can have a look at the release notes for each revision. For MySQL 5.6, the release notes are stored at the following location:
http://dev.mysql.com/doc/relnotes/mysql/5.6/en/
At the time I write this, the topmost entry is a link for MySQL 5.6.15. Each linked page shows the changes in that particular version, so if you are currently running 5.6.11 and are interested in what bugs were fixed in versions since then, you can look at each subsequent release and see all changes in glorious detail.
One really clever thing you can do with that site is do an advanced Google search to find exactly when a feature was released, and find out its release notes. By using the preceding link in a "site:" directive in Google, you can search only within those pages for an entry. For example, the following Google search shows pages within the release notes that reference the --slow-start-timeout option:
site:http://dev.mysql.com/doc/relnotes/mysql/ "--slow-start-timeout"
By running that search, you can see that the option was added in MySQL 5.6.5 and also rolled into MySQL 5.5.20.
White Papers
Also, with each major release you can usually find a white paper describing what's new in that release. In MySQL 5.6 there was a "What's new" whitepaper at this location: http://www.mysql.com/why-mysql/white-papers/whats-new-mysql-5-6/
You'll find other white papers at:
http://www.mysql.com/why-mysql/white-papers/
Search the page for "5.6" to see any papers dealing specificallly with that version.
