Submit open-mastr paper to joss

[Florian Kotthoff]

For a paper submission, we need to do the following:

• Create a branch containing a paper.md file
• Release v0.14.0
• Update the documentation to mkdocs
• Conduct an own review of open-mastr, following the checklist described here
• Run the whole paper.md file through a spell checker
• Get permission from all authors to submit paper

Tasks within the paper

• Add orcids of all authors
• Add affiliation of all authors

[Florian Kotthoff]

Paper is submitted - search for editors and reviewers has started, see openjournals/joss-reviews#6199 (comment)