update support.md

[jputrino]

new text:

Maintenance and F5 Technical Support of the F5 code is provided only if the software (i) is unmodified; and (ii) has been marked as F5 Supported in SOL80012344, (https://support.f5.com/kb/en-us/solutions/public/k/80/sol80012344.html). Support will only be provided to customers who have an existing support contract, purchased separately, subject to F5’s support policies available at http://www.f5.com/about/guidelines-policies/ and http://askf5.com.