clarify security.md document

[martenson]

          It might be beneficial to clarify the security commitment wrt what has been added here and also the release versioning. e.g. the following seems confusing now:

galaxy/SECURITY.md

Lines 13 to 20 in 8adfb7e

	## Supported versions
	The following branches or releases receive security support:
	- Development on the `dev` branch, hosted on GitHub, which will become the next release of Galaxy
	- Releases within the past 12 months.
	- E.g. 16.04 will receive support until 2017-04. As the month changes to 2017-05 it will become unsupported.
	- There are currently no plans for Long Term Support (LTS) releases.

Originally posted by @martenson in #17680 (comment)