Clarify readme (the-benchmarker#230)

* how to add a framework

* be more contributor friendly

Author: waghanza

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+## How to contribute ?
+
+Contributions of any kind a :heart: accepted
+
++ Adding new frameworks
++ Fix some frameworks
++ Update dependencies
++ Discuss best practices
+
+## Adding a framework
+
++ All frameworks **SHOULD** follow this rules :
+
+| HTTP   | Route       | Status code | Response body         |
+| ------ | ----------- | ----------- | --------------------- |
+| `GET`  | `/`         | `200`       | **Empty**             |
+| `GET`  | `/user/:id` | `200`       | **id** given as param |
+| `POST` | `/user`     | `200`       | **Empty**
+
++ All framework **SHOULD** contain a `Dockerfile`
+
++ All framework **SHOULD** be referenced in :
+   + `Makefile`, a target group for the language, and a target for the framework
+   + `neph.yaml`, a target group for the language, and a target for the framework
+   + `benchmarker.cr`, a hash for language containing all frameworks an repository information