Update CONTRIBUTING.md

Improved readability & fixed spelling.

CONTRIBUTING.md

@@ -1,11 +1,12 @@
 ## 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.
++ If you want to **fix bugs** or **implement new features** that **already have an [issue card](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.
++ If you want to **implement a new feature:** 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.
++ **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).
++ 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:
+**Refactoring** or some **dev-faces improvements** might also 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).
++ **Performance matters.** In the case of choosing between source code beauty and performance, performance should be a priority. 
-- Avoid adding new dependencies unless required. APK size is important.
++ Please, **do not modify readme and other information files** (except for typos).
++ **Avoid adding new dependencies** unless required. APK size is important.