Create CONTIRBUTING.md

[loreloc]

Create CONTIRBUTING.md (or another name) and put useful information/documentation and standardizations.
For example, the tensor dimension names in #112 or the docstring format in #47 .

[lkct]

also:
#205 (though partly for contributors, partly for maintainers)
#152 for raises
#81 for import style

[andreasgrv]

• Use specific exceptions instead of asserts when testing for properties of classes