Move mention of specific limit of redirects to Security Considerations section

[aaronpk]

Change:

it must perform an HTTP GET request on source, and follow any HTTP redirects (up to a self-imposed limit such as 20) and confirm that it actually links to the target.

to

it must perform an HTTP GET request on source, following any HTTP redirects (and SHOULD limit the number of redirects it follows) to confirm that the target URL actually links to the target.

The suggested limit of 20 should be moved to the "Limits on GET requests" section.

[aaronpk]

This is completed