DSU-1270 Add contributing guide (mailgun#138)

CONTRIBUTING.md

@@ -0,0 +1,29 @@
+# Contributing to holster
+
+## Go version
+Use the same version as in the [go.mod](https://github.com/mailgun/holster/blob/master/go.mod#L3) file.
+
+## go.mod and go.sum
+`go mod tidy` should be called before commit to add missing and remove unused modules.
+
+## Formatting
+Prior to commit all code should be formatted by the [goimports](https://pkg.go.dev/golang.org/x/tools/cmd/goimports) tool.
+
+### How to configure IDEs
+#### GoLand
+Go to Preferences → Editor → Code style → Go → Imports → Sorting, choose "goimports".
+
+#### VSCode
+1. Call `go install golang.org/x/tools/cmd/goimports@latest`.
+1. Install Golang extension.
+1. Go to Settings → Extensions → Format tool, choose "goimports".
+
+#### Emacs, Vim, GoSublime, etc.
+See [goimports doc](https://pkg.go.dev/golang.org/x/tools/cmd/goimports?tab=doc).
+
+## Linters
+We are using [golangci-lint](https://golangci-lint.run/) for static code analysis.
+
+In case of `Lint / lint` GitHub Action failure check the annotations on the PR’s `Files changed` tab.
+
+You could also run `make lint` on your local env to check it.

README.md

@@ -588,3 +588,6 @@ See package directory `ctxutil`.
 Use functions `ctxutil.WithDeadline()`/`WithTimeout()` instead of the `context`
 equivalents to log details of the deadline and source file:line where it was
 set.  Must enable debug logging.
+
+## Contributing code
+Please read the [Contribution Guidelines](./CONTRIBUTING.md) before sending patches.