Doclinks added to contribution guide

CONTRIBUTING.md

@@ -67,6 +67,26 @@ WARNING! Because of the current patching solution, `make doc` modifies some of t
 so it is strongly advised to use it carefully and in virtual environment only.
 However, this behavior is likely to be changed in the future.
 
+#### Documentation links
+
+In your tutorials, you can use special expanding directives in markdown cells.
+They can help shorten the comments and avoid boilerplate code.
+The documentation links generated by the directives are always relative
+to the local documentation and verified during build.
+
+- `%pip install {args}`
+    This directive generates dependency installation cell, adds a comment and sets up "quiet" flag.
+
+    It should be used in tutorials, like this: `# %pip install dff[...]`.
+- `%doclink({args})`
+    This directive generates a documentation link. It supports 2 or three arguments and the generated link will look like: `ARG1/ARG2#ARG3`.
+
+    The first argument can be either `api` for DFF codebase, `tutorial` for tutorials or `guide` for user guides.
+- `%mddoclink({args})`
+    This directive is a shortcut for `%doclink` that generates a markdown format link instead.
+
+    The generated link will be either `[ARG2](%doclink(ARG1,ARG2))` or `[ARG3](%doclink(ARG1,ARG2,ARG3))`.
+
 ### Style
 For style supporting we propose `black`, which is a PEP 8 compliant opinionated formatter.
 It doesn't take previous formatting into account. See more about [black](https://github.com/psf/black).