doc (contributing): first shot at the contributor guide

CONTRIBUTING.md

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+# Contributing Guide
+
+Thanks for taking the time to join our community and start contributing. This guide will help you get started with the Filestash project.
+
+## How to contribute?
+
+### Before you submit a pull request
+
+For anything else than a typo or an obvious bug fix, please raise an issue to discuss your proposal before submitting any code.
+
+### License for contributions
+
+As the copyright owner, you agree to license your contributions under an irrevocable MIT license.
+
+
+## Building from source
+
+### Prerequisites
+- Git
+- Make
+- Node
+- Go
+
+### Download the source
+```
+export $GOPATH=/wherever/you/want
+cd $GOPATH
+mkdir -p github.com/mickael-kerjean/ && cd github.com/mickael-kerjean/
+git clone https://github.com/mickael-kerjean/filestash
+cd filestash
+```
+
+### Install dependencies
+```
+# frontend dependencies
+npm install
+
+# backend dependencies
+go get ./server/...
+make build_init # beware this will install the required C headers under /usr/local/include/
+```
+
+### Building
+*Frontend*:
+```
+# Production build:
+make build_frontend
+
+# Development build:
+npm run dev
+```
+
+*Backend*:
+```
+make build_backend
+```
+
+*Run*:
+```
+./dist/filestash
+```
+
+*Clear*:
+```
+# clear the frontend
+rm -rf ./dist/data/public/
+# clear the entire build
+rm -rf ./dist
+```
+
+### Tests
+Our tests aren't open source. This comes as an attempt to restrict opportunistic forks (see [1](https://news.ycombinator.com/item?id=17006902#17009852) and [2](https://www.reddit.com/r/selfhosted/comments/a54axs/annoucing_jellyfin_a_free_software_fork_of_emby/ebk92iu/?utm_source=share&utm_medium=web2x)) from creating a stable release without serious commitment and splitting the community in pieces while I'm on holidays. Also the project welcome serious and willing maintainers.