chore: prepare release notes

Author: Midnighter

release-notes/0.20.0.md

@@ -0,0 +1,23 @@
+# Release notes for cobrapy 0.20.0
+
+## New features
+
+* There is a new function inspired by
+  [cameo](https://github.com/biosustain/cameo/) that can load models directly
+  from [BiGG](http://bigg.ucsd.edu/) or
+  [BioModels](https://www.ebi.ac.uk/biomodels/) and by default will cache them
+  locally (#995), for example,
+
+    ```py
+    from cobra.io import load_model
+
+    ecoli = load_model("iML1515")
+    ylipo = load_model("MODEL1604280017")
+    ```
+
+## Fixes
+
+* Over a series of pull requests, @synchon has been modernizing parts of the
+  codebase for Python 3.6+ (#993, #999, #1000, #1003)
+* Upgrade the dependency python-libsbml-experimental to 5.18.3
+