add a MANIFEST.in file (#72)

This looks good for now -- other problems to be solved, but this will get a reasonable release out.

Author: ocefpaf

MANIFEST.in

@@ -0,0 +1,15 @@
+include *.txt
+include README.md
+include pyproject.toml
+
+graft xarray_subset_grid
+
+prune .github
+prune *.egg-info
+prune docs
+prune tests
+
+exclude .gitignore
+exclude .pre-commit-config.yaml
+exclude *.yml
+exclude xarray_subset_grid/_version.py