docs: Add basic document on matching rules support for header fields

[m-guesnon-pvotal]

Related Issue or Design Document

Related source code pull request can be found here: ory/oathkeeper#967

Checklist

• I have read the contributing guidelines
  and signed the CLA.
• I have referenced an issue containing the design document if my change
  introduces a new feature.
• I have read the security policy.
• I confirm that this pull request does not address a security
  vulnerability. If this pull request addresses a security vulnerability, I
  confirm that I got green light (please contact
  security@ory.sh) from the maintainers to push
  the changes.
• I have added tests that prove my fix is effective or that my feature
  works.
• I have added necessary documentation within the code base (if
  appropriate).

Further comments

[CLAassistant]

All committers have signed the CLA.

[vinckr]

added the upstream label to make clear this is waiting for the PR in oathkeeper.