Add contributing guidelines

CONTRIBUTING.md

+# Contributing to SMAM
+
+Contributions are more than welcome! There are several ways to get involved:
+
+## Bug reporting
+
+If you notice a bug while using SMAM, please open an [issue](https://github.com/babolivier/smam/issues) giving as much info as you can on your use of SMAM (without giving sensitive data, such as your full config file) that could be relevant for fixing this it.
+
+## Translation
+
+Since [v2.0.0](https://github.com/babolivier/smam/tree/v2.0.0), SMAM can be used in different languages depending on its configuration. However, it is still limited to the languages for which it has translations!
+
+If you want to help out by prividing translations in a language you know, you can do so by forking this repo, copying the file at [`locales/en.json`](https://github.com/babolivier/smam/blob/master/locales/en.json) somewhere, and translating the strings (only the ones on the right of the colon (:), the ones on the left with underscores (\_) are identifiers). If you haven't already done it, move and/or rename your file so it is located in the `locales` directory and is named `xx.json`, `xx` being the two-letter code for the language.
+
+Then send us your new translation by opening a [pull request](https://github.com/babolivier/smam/pulls)!
+
+## Code
+
+If you want to fix a bug, implement a cool feature or add some optimisation to SMAM, feel free to open a [pull request](https://github.com/babolivier/smam/pulls)!
+
+However, if you feel like you're up against a huge task, or just doesn't know where to start, don't hesitate to [open an issue](https://github.com/babolivier/smam/issues) so a SMAM contributor can help you out :smile:
+
+## Anything else
+
+The beauty of free software is that everyone can contribute in their own way! If you want to share a contribution that doesn't fit in the cases mentionned above (e.g. improvements on the contribution process, sharing a blog post about SMAM, etc), please [open an issue](https://github.com/babolivier/smam/issues)! It's the best way to get your stuff noticed and to start a conversation about it :grin:
+
+# Licensing
+
+SMAM is provided under the [AGPL v3](https://github.com/babolivier/smam/blob/master/LICENSE) license. Every contribution adding up to SMAM's repository must be released under this license.
+