DOC: Add note to update version attribute in releasing process

Author: ev-br

docs/dev/releasing.md

@@ -100,6 +100,11 @@
   docs update (the docs are published automatically from the sources on
   `main`).
 
+- [ ] **Bump the `__version__` attribute in `__init__.py`**
+
+ After an M.N.0 release, further development is towards version `M.(N+1).0`, thus the main branch's 
+ version is `M.(N+1).0.dev0`.
+
 - [ ] **Update conda-forge.**
 
   After the PyPI package is published, the conda-forge bot should update the