contributing: initial doc for contribution guidance (#68)

Summary:
Pull Request resolved: https://github.com/facebookincubator/ent/pull/68

Closes #62

Reviewed By: alexsn

Differential Revision: D17782435

fbshipit-source-id: c2995dbf2c1cbdb032cf4c2865769ffa7a3214ad

CONTRIBUTING.md

@@ -2,6 +2,50 @@
 We want to make contributing to this project as easy and transparent as
 possible.
 
+# Project structure
+
+- `dialect` - Contains SQL and Gremlin code used by the generated code.
+  - `dialect/sql/schema` - Auto migration logic resides there.
+
+- `schema` - User schema API.
+  - `schema/{field, edge, index}` - provides schema builders API.
+  - `schema/field/gen` - Templates and codegen for numeric builders.
+
+- `entc` - Codegen of `ent`.
+  - `entc/load` - `entc` loader API for loading user schemas into a Go objects at runtime.
+  - `entc/gen` - The actual code generation logic resides in this package (and its `templates` package).
+  - `integration` - Integration tests for `entc`.
+  
+- `doc` - Documentation code for `entgo.io` (uses [Docusaurus](https://docusaurus.io)).
+  - `doc/md` - Markdown files for documentation.
+  - `doc/website` - Website code and assets.
+
+# Working on `entc`
+
+If you work on one of the template files (`gen/templates`, `load/templates`, `load/schema.go`), you should
+run `go-bindata`, in order to embedded these files in `entc` binary. Please install it by running the following
+command in your `$HOME` directory.
+
+```
+go get -u github.com/go-bindata/go-bindata
+```
+
+# Run integration tests
+If you touch any file in `entc`, run the following command in `entc/integration`:
+
+```
+go generate ./...
+```
+
+Then, run `docker-compose` in order to spin-up all database containers:
+
+```
+docker-compose -f compose/docker-compose.yaml up -d --scale test=0
+```
+
+Then, run `go test ./...` to run all integration tests.
+
+
 ## Pull Requests
 We actively welcome your pull requests.