add rtd files

[ktactac-ornl]

Short description of the changes:

Sets up readthedocs

Long description of the changes:

Check list for the pull request

• I have read the [CONTRIBUTING]
• I have read the [CODE_OF_CONDUCT]
• I have added tests for my changes
• I have updated the documentation accordingly

Check list for the reviewer

• I have read the [CONTRIBUTING]
• I have verified the proposed changes
• best software practices
  • all internal functions have an underbar, as is python standard
  • clearly named variables (better to be verbose in variable names)
  • code comments explaining the intent of code blocks
• All the tests are passing
• The documentation is up to date
• code comments added when explaining intent

Manual test for the reviewer

References

[jmborr]

can you add in the README.md file, under developer's corner, how to manually build the docs by invoking make html?

[jmborr]

🎉 👌