docs: update readme and contributing rules

CONTRIBUTING.md

@@ -1,7 +1,34 @@
-# Contribution
+# Contributing
 
-To support this project you can:
-  - Post thoughts about new features/optimizations that important to you
-  - Submit bug using one of following patterns:
-    * Java code examples which decompiles incorrectly
-    * Error log and link to _public available_ apk file or app page on Google play
+Please note we have a [code of conduct](CODE_OF_CONDUCT.md), please follow it in all your interactions with the project.
+
+## Open Issue
+
+1. Before proceed please do:
+    - check [latest unstable build](https://bintray.com/skylot/jadx/unstable/_latestVersion#files) (maybe issue already fixed)
+    - check [Troubleshooting Q&A](https://github.com/skylot/jadx/wiki/Troubleshooting-Q&A) section on wiki
+    - search existing issues by exception message
+
+2. Describe error
+    - provide full name of method or class with error
+    - provide full java stacktrace
+
+      **Note**: no need to copy method fallback code (commented pseudocode)
+    - attach or provide link to apk file (double check apk version)
+
+      **Note**: GitHub don't allow attach files with `.apk` extension, but you can change extension by adding `.zip` at the end :)
+
+
+## Pull Request Process
+
+1. Please don't submit any code style fixes, dependencies updates or other changes which are not fixing any issues.
+
+1. Before open a PR please discuss the change you wish to make via issue. PR without corresponding issue will be rejected.
+
+1. Use only features and API from Java 8 or below.
+
+1. If possible don't add additional dependencies especially if they are big.
+
+1. Make sure your code is correctly formatted, see description here: [Code Formatting](https://github.com/skylot/jadx/wiki/Code-Formatting).
+
+1. Make sure your changes is passing build: `./gradlew clean build dist`