Add SECURITY.md file

[RamReso]

IF YOU DON'T REMOVE THESE FOUR LINES, THEN YOUR REQUEST WILL BE CLOSED AUTOMATICALLY
Questions and user problems should be directed at the forum (http://discuss.ardupilot.org)
Please do a careful search before opening this, there are already a lot of feature requests
If there was a previous discussion in the forum, link to it

Description

At the moment being it is not clear what is the best way to disclose a security issue. I propose to add SECURITY.md.

[amilcarlucas]

Please open a pull request with the file