Add developer documentation for packages

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

doc/administration/packages.md

@@ -11,6 +11,10 @@ The Packages feature allows GitLab to act as a repository for the following:
 | [Maven Repository](../user/project/packages/maven_repository.md) | The GitLab Maven Repository enables every project in GitLab to have its own space to store [Maven](https://maven.apache.org/) packages. | 11.3+ |
 | [NPM Registry](../user/project/packages/npm_registry.md)   | The GitLab NPM Registry enables every project in GitLab to have its own space to store [NPM](https://www.npmjs.com/) packages. | 11.7+ |
 
+Don't you see your package management system supported yet? 
+Please consider contributing
+to GitLab. This [development documentation](../development/packages.md) will guide you through the process.
+
 ## Enabling the Packages feature
 
 NOTE: **Note:**

doc/development/packages.md

+# Packages
+
+This document will guide you through adding another [package management system](../administration/packages.md) support to GitLab.
+
+See already supported package types in [Packages documentation](../administration/packages.md)
+
+Since GitLab packages' UI is pretty generic, it is possible to add new
+package system support by solely backend changes. This guide is superficial and does 
+not cover the way the code should be written. However, you can find a good example 
+by looking at existing merge requests with Maven and NPM support: 
+
+- [NPM registry support](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8673). 
+- [Maven repository](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6607).
+- [Instance level endpoint for Maven repository](https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8757)
+
+## General information
+
+The existing database model requires the following:
+
+- Every package belongs to a project. 
+- Every package file belongs to a package.
+- A package can have one or more package files.
+- The package model is based on storing information about the package and its version.
+
+## API endpoints
+
+Package systems work with GitLab via API. For example `ee/lib/api/npm_packages.rb` 
+implements API endpoints to work with NPM clients. So, the first thing to do is to 
+add a new `ee/lib/api/your_name_packages.rb` file with API endpoints that are 
+necessary to make the package system client to work. Usually that means having 
+endpoints like: 
+
+- GET package information.
+- GET package file content.
+- PUT upload package.
+
+Since the packages belong to a project, it's expected to have project-level endpoint
+for uploading and downloading them. For example: 
+
+```
+GET https://gitlab.com/api/v4/projects/<your_project_id>/packages/npm/
+PUT https://gitlab.com/api/v4/projects/<your_project_id>/packages/npm/
+```
+
+Group-level and instance-level endpoints are good to have but are optional. 
+
+NOTE: **Note:**
+To avoid name conflict for instance-level endpoints we use 
+[the package naming convention](../user/project/packages/npm_registry.html#package-naming-convention)
+
+## Configuration
+
+GitLab has a `packages` section in its configuration file (`gitlab.rb`). 
+It applies to all package systems supported by GitLab. Usually you don't need 
+to add anything there. 
+
+Packages can be configured to use object storage, therefore your code must support it. 
+
+## Database
+
+The current database model allows you to store a name and a version for each package.
+Every time you upload a new package, you can either create a new record of `Package`
+or add files to existing record. `PackageFile` should be able to store all file-related
+information like the file `name`, `side`, `sha1`, etc.
+
+If there is specific data necessary to be stored for only one package system support, 
+consider creating a separate metadata model. See `packages_maven_metadata` table 
+and `Packages::MavenMetadatum` model as example for package specific data.