[contributionguide] Description of backwards compatibility policy

CONTRIBUTING.md

@@ -59,6 +59,68 @@ If you have any questions or complaints about this workflow of contributing to D
 * Merge requests get reviewed by at least one main developer.
 * If continuous integration (GitLabCI / BuildBot) is enabled for merge requests, the pipeline has to pass before merging anything to master.
 
+## Backwards Compatibility
+As a general rule, all changes added to the dumux master version should be made 
+such that:
+*  all tests and modules using dumux will still compile, and
+*  the user is warned at compile time of any interface changes.
+
+This can be done by masking removed methods with deprecation warnings: (example 1)
+
+```c++
+[[deprecated("This method will be removed after release (3.n). Use newMethod(x,y,z) instead!")]]
+int oldMethod(const int x, const int y) const
+{ return x + y; }
+
+int newMethod(const int x, const int y, const int z) const
+{ return x + y > z ? x + y : z; }
+``` 
+
+or by adding intermediate method calls with warnings guarded by isValid 
+in the [`dumux/common/deprecated.hh`](dumux/common/deprecated.hh) header: (example 2)
+
+```c++
+// support old interface of the coolMethod() function on problems
+template<class B>
+struct HasNewCoolMethodIF
+{ template<class A> auto operator()(A&& a) -> decltype( a.coolMethod(std::declval<const B>()) ) {} };
+
+template<class A, class B
+         typename std::enable_if_t<!decltype(isValid(HasNewCoolMethodIF<B>()).template check<A>())::value, int> = 0>
+auto DUNE_DEPRECATED_MSG("Use new coolMethod() interface that additionally receives 
+                          the object b! This will be removed after 3.n release")
+coolMethod(const A a, const B b)
+{ return a.coolMethod(); }
+
+template<class A, class B,
+         typename std::enable_if_t<decltype(isValid(HasNewCoolMethodIF<B>()).template check<A>())::value, int> = 0>
+auto coolMethod(const A a, const B b)
+{ return a.coolMethod(b); }
+
+```
+and replace the call with: 
+
+``` c++
+Deprecated::template coolMethod(a, b); //TODO: Replace this after release 3.n
+
+```
+
+There are other methods for deprecating old interfaces, please see [ (cppref/deprecated) ](https://en.cppreference.com/w/cpp/language/attributes/deprecated) for more information.
+
+In addition, please be sure to:
+
+*  mark all deprecated interfaces with the release after which the deprecated interface will be removed, and
+*  add a detailed description of the changes made to the [changelog](CHANGELOG.md)
+
+**In some more complicated cases,** guaranteeing backwards compatibility for all possible 
+cases is not feasible, or would require enormous intrusive changes.  In this case, we recommend you do the following:
+1.  Organize all changes neatly in a merge request.
+2.  Include a detailed description of the changes in the changelog.
+3.  Within the comments section of the merge request, mark one of the core developers,
+ and document the reasons why guaranteeing backwards compatibility would not be feasible, and which cases will likely be effected. 
+4.  The core developers will decide if the changes and efforts are sufficient during the next monthly core developers meeting (aka DumuxDay). 
+5.  Should your efforts be deemed sufficient, continue with the standard MR procedure. 
+
 ## Patches
 
 * Patches can be supplied via the mailing list,