modules should be open source

.github/CONTRIBUTING.md

@@ -23,6 +23,8 @@ If you want to contribute some large change, please consider adding a new module
 Documentation about adding new modules is still limited, but you can get inspiration
 from the existing ones. This procedure will allow you to keep the ownership on your code.
 On the other hand, we expect that you will maintain it in the future.
+In order to incorporate a new module into the main repository, we ask contributors to declare that
+the module is available with an open source license.
 
 Finally, notice that you can always share modified versions of PLUMED with your changes.
 We are happy if you want to host on github a fork of PLUMED with additional features.

.github/PULL_REQUEST_TEMPLATE.md

@@ -35,7 +35,7 @@ I would like my code to appear in release __XXXXX__
   MAKE SURE TO TICK ALSO THE FOLLOWING BOX
 -->
 
-- [ ] the module I added or modified contains a `COPYRIGHT` file with the correct license information. I also used the command `cd src && ./header.sh mymodulename` in order to make sure the headers of the module are correct.
+- [ ] the module I added or modified contains a `COPYRIGHT` file with the correct license information. Code should be released under an open source license. I also used the command `cd src && ./header.sh mymodulename` in order to make sure the headers of the module are correct. 
 
 ##### Tests