[doc] Add preliminary contribution guide and style guide

CONTRIBUTING.md

+# Contribution guidelines for DuMu<sup>x</sup>
+
+## Style guide
+When contributing code to DuMu<sup>x</sup> please follow the [styleguide](doc/styleguide.md).

doc/styleguide.md

+# Style guide
+
+## File system
+
+* Try to order your new header into the existing directory structure
+* Headers are named like the classes they contain (usually one class per file, exception: closely tied helper classes / functions)
+* Headers are names lower case only (using underscores only if absolutely necessary for readability)
+* Folder names are lower case only
+* Tests should be called after model and discretization scheme using underscores and lower case only, e.g.
+
+__Example:__
+```
+test_1p_tpfa
+test_2p2c_box_infiltration
+```
+
+## C++
+
+### Indent
+
+* The default indent is 4 spaces (no tabs, not 2 spaces)
+
+### Formatting
+
+* Curly brackets that open or close a scope are on a separate line (except for [namespaces](#Namespaces))
+* There should be a space between `if`,`else if`,`switch` and the condition
+* `if`,`else if`,`switch` can omit brackets if the expression is only one line
+
+```c++
+// comment for if block, space between if and (enableGravity)
+if (enableGravity)
+{
+    Scalar b = 1.0;
+    return b*gravity_[dimWorld-1];
+}
+
+// comment for else-block
+else
+    return 1.0; // ok, one-liner
+```
+
+### Namespaces
+
+* Open curly brackets on the same line
+* Do not indent the code inside the namespace
+* Comment closing curly brackets uniquely
+__Example:__
+
+```c++
+namespace Dumux {
+namespace Properties {
+
+bool here = true;
+    bool nothere = false; // not like this
+
+} // end namespace Properties
+} // end namespace Dumux
+```
+
+### Includes
+
+* Space between `#include` and path
+* C++ standard library includes first, then Dune, then others, then DuMu<sup>x</sup>
+* Always use project relative paths
+    - exception: the other header is in the same folder and closely related (`#include "volumevariables.hh"`)
+
+```c++
+#include <type_traits>
+#include <dune/common/fvector.hh>
+#include <dumux/common/exceptions.hh>
+```
+
+### Documentation
+
+* Prefer long-style doxygen documentation of the form
+* at least containing a `\brief`
+
+```c++
+/*
+ * \brief Base class for all linear solvers
+ */
+```
+
+* document all parameters of functions and what it returns
+* optionally document which error the function might throw
+
+### Classes
+
+TODO
+
+#### Member functions
+
+TODO
+
+#### Private data member
+* Use underscore postfix
+
+```c++
+...
+private:
+    int steps_;
+...
+```
+
+### Functions
+
+TODO
+
+### Property system
+
+* Prefer class templates with regular template arguments over class templates with a `TypeTag` as template argument
+
+TODO