added draft paper.md

[chiaral]

This PR creates a draft for the JOSS paper.

[codecov]

Codecov Report

Merging #35 into master will decrease coverage by 0.80%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #35      +/-   ##
==========================================
- Coverage   67.01%   66.20%   -0.81%     
==========================================
  Files           9        9              
  Lines         288      290       +2     
  Branches       76       76              
==========================================
- Hits          193      192       -1     
- Misses         73       76       +3     
  Partials       22       22              

Impacted Files	Coverage Δ
src/xcape/cape_numba.py	0.00% <0.00%> (-50.00%)	⬇️
src/xcape/core.py	83.33% <0.00%> (-1.09%)	⬇️
src/xcape/srh.py	93.33% <0.00%> (ø)
src/xcape/cape_fortran.py	63.63% <0.00%> (ø)

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