Merge pull request #388 from lorenzwalthert/contributing

- More on contributing advice (#388).

Author: lorenzwalthert

CONTRIBUTING.md

@@ -92,6 +92,25 @@ The source code is organized as follows:
 | visit.R | Functions that apply functions to each level of nesting, either inside out or outside in. | 
 | zzz.R | backport imports. |
 
+## Obtaining contextual information
+
+You may have problems understanding some code because documentation is minimal,
+some code / functions seem to solve problems you don't understand or handle
+cases that seem unreasonable or otherwise incomprehensible. You can resort to 
+the following strategies:
+
+* Use full-text search to see where functions are defined or called and how
+  different parts of styler depend on it.
+* Use `$git blame` to see where changes were introduced. Look at the commit
+  message, check changes that were made to the code in the same commit. If you
+  are using the GUI of GitHub, you can easily obtain more contextual information
+  such as the pull request with which a change was introdued. Often 
+  functionality was introduced with testing. So you can easily see which new 
+  tests are related to the new functionality. You can remvoe the changes in the 
+  source code and re-run the tests and see what fails and why.
+* Search Issues and Pull Requests on GitHub with the full text search. Make 
+  sure you also search for closed Issues and PRs.
+
 
 ## High-level conventions