Readme updates concerning versions

[jrcain-usgs]

Description

Small readme update concerning (legacy) version and releases, and RFCs. Note, info regarding legacy ISIS versions in response to the issue have been PRed to ASC Docs ISIS FAQs, rather than this readme directly.

Sister PR to ASC Docs updating the FAQ: DOI-USGS/asc-public-docs#135

Related Issue

#5691

How Has This Been Validated?

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Documentation change (update to the documentation; no code change)
• Breaking change (fix or feature that would cause existing functionality to change)
• Infrastructure change (changes to things like CI or the build system that do not impact users)

Checklist:

• I have read and agree to abide by the Code of Conduct
• I have read the CONTRIBUTING document.
• My change requires a change to the documentation and I have updated the documentation accordingly.
• I have added tests to cover my changes.
• I have added myself to the .zenodo.json document.
• I have added my user impacting change to the CHANGELOG.md document.

Licensing

This project is mostly composed of free and unencumbered software released into the public domain, and we are unlikely to accept contributions that are not also released into the public domain. Somewhere near the top of each file should have these words:

This work is free and unencumbered software released into the public domain. In jurisdictions that recognize copyright laws, the author or authors of this software dedicate any and all copyright interest in the software to the public domain.

• I dedicate any and all copyright interest in this software to the public domain. I make this dedication for the benefit of the public at large and to the detriment of my heirs and successors. I intend this dedication to be an overt act of relinquishment in perpetuity of all present and future rights to this software under copyright law.

[github-actions]

The build and test suite have started for your pull request.

To view your build log, please reference the build with source version: "PR_5739".

Additionally, check the latest "dev" source version to identify existing test failures. Please note that you are not responsible for the test failures that exist on both your PR and the dev branch.