Update github pull request template.

* Update CONTRIBUTING.md guide with hints and examples.
* Minor GitHub Pull Request Template update hinting expected information.

Signed-off-by: Tomasz 'CeDeROM' CEDRO <[email protected]>

Author: cederom

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,6 +1,21 @@
+*Note: Please adhere to [Contributing Guidelines](../CONTRIBUTING.md).*
+
 ## Summary
 
+*Update this section with information on why change is necessary,
+ what it exactly does and how, if new feature shows up, provide
+ references (dependencies, similar problems and solutions), etc.*
+
 ## Impact
 
+*Update this section, where applicable, on how change affects users,
+ build process, hardware, documentation, security, compatibility, etc.*
+
 ## Testing
 
+*Update this section with details on how did you verify the change,
+ what Host was used for build (OS, CPU, compiler, ..), what Target was
+ used for verification (arch, board:config, ..), etc. Providing build
+ and runtime logs from before and after change is highly appreciated.*
+
+