Add developer's guide (#4)

* KAMUDEP-26 | Developer's guide

* Some improvements

DEVELOPERS.md

@@ -0,0 +1,53 @@
+# Developer's Guide
+
+
+## Repository content and structure
+
+This repository contains all charts published by Kamu. In general
+they could be classified into two categories:
+
+* **Product charts** manage different components of Kamu platform. They
+  can be installed independently and should be considered production ready.
+* **Service charts** are used to either perform different service operations,
+  running tests or to manage dependencies of Kamu in local or demo environments.
+  These charts are opinionated and shall not be used in production.
+
+Each chart is stored in its corresponding directory in `charts`. The repository
+contains only the sources of the charts. Packages are built and published
+automatically within the CI/CD pipeline.
+
+
+## Introducing changes to charts.
+
+The `master` branch is the source of truth for all released versions of each chart.
+For that reason contributors must stick to the following rules:
+
+* Every change requires an approved pull request before being merged.
+* Changes to charts should be done along with the version bump.
+* **TODO**: Linting tests must pass before a change can be merged.
+
+
+## Releasing charts
+
+Charts are released automatically after every merge to master by a
+[GitHub workflow][1]. The workflow uses [Chart Releaser action][2] to manage
+the GitHub Pages and to create GitHub releases in the repository. It
+automatically updates the `index.yaml` file, creates tags, builds charts
+and uploads binaries together with the changelog.
+
+
+## Publishing charts and making the repository available
+
+The repository index is published along with the landing page as a GitHub
+Pages website. GitHub's embedded workflow automatically generates the website
+every time the release workflow pushes new changes to the `gh-pages` branch.
+The `README.md` file is the source for the landing page.
+
+The index file references chart packages that are published as GitHub releases.
+ArtifactHub [to publish][3] scans the index file periodically to publish
+all charts' metadata and living documentation to users.
+
+
+[1]: https://github.com/kamu-data/helm-charts/blob/master/.github/workflows/release.yml
+[2]: https://helm.sh/docs/howto/chart_releaser_action/
+[3]: https://artifacthub.io/packages/search?repo=kamu&sort=relevance&page=1

TODO.md

@@ -1,4 +1,3 @@
-- setup repository (ghpages?)
 - setup CI (linting,  testing, release)
 - chart.lock
 - reuse macros across charts