add links, improve wording

Author: vladak

CONTRIBUTING.md

@@ -3,14 +3,14 @@ the general direction of the project, feel free to submit new Issue first and le
 
 Please follow pre-existing coding style.
 
-Asking questions via creating new Issue is fine also.
+Asking questions via creating new [discussion](https://github.com/oracle/opengrok/discussions) is fine.
 
 When submitting a new Issue for what seems is a bug, please note the versions (OpenGrok, Tomcat, Universal ctags etc.) you're running and ideally steps to reproduce.
 
-Make sure to add a comment line line to the changeset saying which Issue it is is fixing, e.g. `fixes #XYZ` so that the issue can be automatically closed when the associated pull request is merged.
+Make sure to add a comment line to the changeset saying which Issue it is is fixing, e.g. `fixes #XYZ` so that the issue can be automatically closed when the associated pull request is merged.
 
 Each pull request should be accompanied by a comment explaining how the change was tested, unless unit tests are provided/updated.
-Please make every effort to make most of your changes covered by tests.
+Please make every effort to make most of your changes covered by tests. Also, for more complicated changes, please make any effort to explain the rationale behind the changes (providing answers to the 'why ?' questions is very important).
 
 Feel free to add a Copyright line to the source files wehre you made non-trivial changes.