docs: move and refactor contributing.md

docs/contributing.md

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+# How to contribute to Capsule
+
+First, thanks for your interest in Capsule, any contribution is welcome!
+
+## Development environment setup
+
+The first step is to set up your local development environment.
+
+Please follow the [Capsule Development Guide](dev-guide.md) for details.
+
+## Code convention
+
+The changes must follow the Pull Request method where a _GitHub Action_ will
+check the `golangci-lint`, so ensure your changes respect the coding standard.
+
+### golint
+
+You can easily check them issuing the _Make_ recipe `golint`.
+
+```
+# make golint
+golangci-lint run -c .golangci.yml
+```
+
+> Enabled linters and related options are defined in the [.golanci.yml file](../../.golangci.yml)
+
+### goimports
+
+Also, the Go import statements must be sorted following the best practice:
+
+```
+<STANDARD LIBRARY>
+
+<EXTERNAL PACKAGES>
+
+<LOCAL PACKAGES>
+```
+
+To help you out you can use the _Make_ recipe `goimports`
+
+```
+# make goimports
+goimports -w -l -local "github.com/clastix/capsule" .
+```
+
+### Commits
+
+All the Pull Requests must refer to an already open issue: this is the first phase to contribute also for informing maintainers about the issue.
+
+Commit's first line should not exceed 50 columns.
+
+A commit description is welcomed to explain more the changes: just ensure
+to put a blank line and an arbitrary number of maximum 72 characters long
+lines, at most one blank line between them.
+
+Please, split changes into several and documented small commits: this will help us to perform a better review. Commits must follow the Conventional Commits Specification, a lightweight convention on top of commit messages. It provides an easy set of rules for creating an explicit commit history; which makes it easier to write automated tools on top of. This convention dovetails with Semantic Versioning, by describing the features, fixes, and breaking changes made in commit messages. See [Conventional Commits Specification](https://www.conventionalcommits.org) to learn about Conventional Commits.
+
+> In case of errors or need of changes to previous commits,
+> fix them squashing to make changes atomic.
+
+### Miscellanea
+
+Please, add a new single line at end of any file as the current coding style.