Sign counter alg clarification [see issue #507] (#539)

* added description for U2F attestation format

* more description added

* ...

* added RP processing rule for signature counter

* explanation added: why should you compare signature counter

* add explanation: where does signature come from

* changes as requested in the PR

* changes as requested in the PR

* changes as requested in the PR

* correction

* more corrections

* typo

* corrected phrase as indicated by equalsJeffH

* correction: bikeshed still wants spaces - not tabs

* updating signcounter consideration according to suggestion in comment

* synatax corrections

* synatax corrections

* more changes added

* more corrections according to the comments in the issue

* addressed second last comment

* removed signing procedure details and referred to CTAP2 section 7

* fixed indent issue

* fix typo