Add contribution guidelines

CONTRIBUTING.md

@@ -0,0 +1,11 @@
+## Kotatsu contribution guidelines
+
+- If you want to fix bug or implement a new feature, that already mention in the [issues](https://github.com/KotatsuApp/Kotatsu/issues), please, assign this issue to you and/or comment about it.
+- Whether you have to implement new feature, please, open an issue or discussion regarding it to ensure it will be accepted.
+- Translations have to be managed using the [Weblate](https://hosted.weblate.org/engage/kotatsu/) platform.
+- In case you want to add a new manga source, refer to the [parsers repository](https://github.com/KotatsuApp/kotatsu-parsers).
+
+Refactoring or some dev-faces improvements are also might be accepted, however please stick to the following principles:
+- Performance matters. In the case of choosing between source code beauty and performance, performance should be a priority. 
+- Please, do not modify readme and other information files (except for typos).
+- Avoid adding new dependencies unless required. APK size is important.

README.md

@@ -39,6 +39,10 @@ Kotatsu is a free and open source manga reader for Android.
 Kotatsu is localized in a number of different languages, if you would like to help improve these or add new languages,
 please head over to the [Weblate project page](https://hosted.weblate.org/engage/kotatsu/)
 
+### Contributing
+
+See [CONTRIBUTING.md](./CONTRIBUTING.md) for the guidelines.
+
 ### License
 
 [![GNU GPLv3 Image](https://www.gnu.org/graphics/gplv3-127x51.png)](http://www.gnu.org/licenses/gpl-3.0.en.html)