Update release document links in README.md

[skliper]

Is your feature request related to a problem? Please describe.
All references in

Additional release notes for the cFS Framework are found in [release notes](https://github.com/nasa/cFE/tree/master/docs/cFE_release_notes.md).  See the [version description document](https://github.com/nasa/cFE/tree/master/docs/cFE_6_6_0_version_description.md) for the full version description document.  Test results can be found in [test results](https://github.com/nasa/cFE/tree/master/test-and-ground/test-review-packages/Results).  This is a point release from major development work currently being performed on an internal repository.

are broken.

Describe the solution you'd like
Fix links - relate to 6.7 release when available.

Describe alternatives you've considered
None

Additional context
None

Requester Info
Jacob Hageman - NASA/GSFC
Email from @dmccomas

[skliper]

Fixed in c7f6d89#diff-04c6e90faac2675aa89e2176d2eec7d8