Add contribution guidelines (#139)

README.md

@@ -69,11 +69,19 @@ Or you could just buy the app from Google Play Store. You'll get constant update
 
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+<h2 align="center">Contributions</h2>
+
+Pull requests are welcome. For major changes, please open an issue first to discuss what you would like to change, or feel free to tackle any of the open issues present at the moment. If you're doing the latter, please leave a comment on the issue you want to contribute towards before starting to work on it.
+
+------
+
 <h2 align="center">Translations</h2>
 
-If you want to make app available in your language, you're very welcome to create a [pull request](https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/proposing-changes-to-your-work-with-pull-requests/about-pull-requests) with your translation file.
-The string resources can be found under `/app/src/main/res/values/strings.xml`. It is easiest to make a translation using the Android Studio XML editor, but you can always go with your favorite XML-text editor instead.
-Checkout this guide to learn more about translation strings from [Helpshift](https://developers.helpshift.com/android/i18n/) for Android.
+If you want to make the app available in your language, you're welcome to create a pull request with your translation file. The base string resources can be found under:
+```
+/app/src/main/res/values/strings.xml
+```
+It is easiest to make a translation using the Android Studio XML editor, but you can always use your favorite XML text editor instead. Check out this guide to learn more about translation strings from [Helpshift](https://developers.helpshift.com/android/i18n/) for Android.
 
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