Update databases.md

run mysql_secure_installation to secure the freshly installed mysql server
Respect group members in mentionable concern
Improve commit diff

* show highlights when replace empty line with content
* show inline diff when replace line with spaces with content
Fix several_namespaces? to work with multiple ownership of groups
Version 6.0.2

Includes next fixes:
* Respect group members in mentionable concern - 626359831402aeede4c4cb12cb10c7534a0dae79
* Improve commit diff - 9109a207589c3a3d085005ee87049bba6aeecc1a
* User cannot select namespace for new project - e19526c2ac3e6a8ae62934c09ac70219062c818a
Fixed the link to the init script

The init script was pointing to the master branch rather than the 6-0-stable causing the rake gitlab:check to think it is out of date.
Merge pull request #5082 from twistedR/6-0-stable

Fixed the link to the init script
Merge pull request #4891 from denics/patch-2

Add mysql_secure_installation to databases.md