a new PR template

closes #110

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,4 +1,17 @@
-# Description of the proposed change
 
-*Please include a short description of enhancement here*
+## Description
 
+> Describe functionality that the PR adds, changes or removes; optionally link
+> issues that are closed by the PR.
+
+#### Checklist
+
+> Feel free to open PRs that do not satisfy the whole checklist. On the other
+> hand, all these are required for merging.
+
+- [ ] All code is formatted with
+  [JuliaFormatter](https://github.com/domluna/JuliaFormatter.jl)
+  (run `using JuliaFormatter; format("path/to/COBREXA.jl");`)
+- [ ] Tests work (try with `]test COBREXA`)
+- [ ] All new functions have proper docstrings
+- [ ] Documentation builds correctly (try with `cd docs; julia make.jl`)