readme: update info on github migration

Provide some information on the migration from gitlab to github
and how to update the git remote url.

README.md

@@ -7,6 +7,8 @@
     https://codecov.io/gl/deltares/rtc-tools
 )
 
+> **NOTE** The rtc-tools repository has been migrated from gitlab to here;
+see [migration from gitlab](#migration-from-gitlab).
 
 RTC-Tools is an open-source Python package for simulation and optimization of cyber-physical systems.  It can be used for model-predictive control and operational decision making of water and energy systems.  It is developed and maintained by Deltares, in collaboration with partners.
 
@@ -38,6 +40,18 @@ To streamline the integration with user interfaces and data management systems (
 RTC-Tools uses [CasADi](https://web.casadi.org/) as a symbolic framework for algorithmic differentiation, as well as for interfacing with numerical optimization solvers.
 
 
+## Migration from GitLab
+
+The rtc-tools repository has been migrated from gitlab (https://gitlab.com/deltares/rtc-tools)
+to here.
+To change the git remote url, run
+
+`git remote set-url origin https://github.com/Deltares/rtc-tools.git`
+
+and
+
+`git remote set-url --push origin https://github.com/Deltares/rtc-tools.git`.
+
 ## Install
 
 ```bash