Use mattermost v5 module (#1192)

vendor/github.com/pelletier/go-toml/CONTRIBUTING.md

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+## Contributing
+
+Thank you for your interest in go-toml! We appreciate you considering
+contributing to go-toml!
+
+The main goal is the project is to provide an easy-to-use TOML
+implementation for Go that gets the job done and gets out of your way –
+dealing with TOML is probably not the central piece of your project.
+
+As the single maintainer of go-toml, time is scarce. All help, big or
+small, is more than welcomed!
+
+### Ask questions
+
+Any question you may have, somebody else might have it too. Always feel
+free to ask them on the [issues tracker][issues-tracker].  We will try to
+answer them as clearly and quickly as possible, time permitting.
+
+Asking questions also helps us identify areas where the documentation needs
+improvement, or new features that weren't envisioned before. Sometimes, a
+seemingly innocent question leads to the fix of a bug. Don't hesitate and
+ask away!
+
+### Improve the documentation
+
+The best way to share your knowledge and experience with go-toml is to
+improve the documentation. Fix a typo, clarify an interface, add an
+example, anything goes!
+
+The documentation is present in the [README][readme] and thorough the
+source code. On release, it gets updated on [GoDoc][godoc]. To make a
+change to the documentation, create a pull request with your proposed
+changes. For simple changes like that, the easiest way to go is probably
+the "Fork this project and edit the file" button on Github, displayed at
+the top right of the file. Unless it's a trivial change (for example a
+typo), provide a little bit of context in your pull request description or
+commit message.
+
+### Report a bug
+
+Found a bug! Sorry to hear that :(. Help us and other track them down and
+fix by reporting it. [File a new bug report][bug-report] on the [issues
+tracker][issues-tracker]. The template should provide enough guidance on
+what to include. When in doubt: add more details! By reducing ambiguity and
+providing more information, it decreases back and forth and saves everyone
+time.
+
+### Code changes
+
+Want to contribute a patch? Very happy to hear that!
+
+First, some high-level rules:
+
+* A short proposal with some POC code is better than a lengthy piece of
+  text with no code. Code speaks louder than words.
+* No backward-incompatible patch will be accepted unless discussed.
+  Sometimes it's hard, and Go's lack of versioning by default does not
+  help, but we try not to break people's programs unless we absolutely have
+  to.
+* If you are writing a new feature or extending an existing one, make sure
+  to write some documentation.
+* Bug fixes need to be accompanied with regression tests.
+* New code needs to be tested.
+* Your commit messages need to explain why the change is needed, even if
+  already included in the PR description.
+
+It does sound like a lot, but those best practices are here to save time
+overall and continuously improve the quality of the project, which is
+something everyone benefits from.
+
+#### Get started
+
+The fairly standard code contribution process looks like that:
+
+1. [Fork the project][fork].
+2. Make your changes, commit on any branch you like.
+3. [Open up a pull request][pull-request]
+4. Review, potential ask for changes.
+5. Merge. You're in!
+
+Feel free to ask for help! You can create draft pull requests to gather
+some early feedback!
+
+#### Run the tests
+
+You can run tests for go-toml using Go's test tool: `go test ./...`.
+When creating a pull requests, all tests will be ran on Linux on a few Go
+versions (Travis CI), and on Windows using the latest Go version
+(AppVeyor).
+
+#### Style
+
+Try to look around and follow the same format and structure as the rest of
+the code. We enforce using `go fmt` on the whole code base.
+
+---
+
+### Maintainers-only
+
+#### Merge pull request
+
+Checklist:
+
+* Passing CI.
+* Does not introduce backward-incompatible changes (unless discussed).
+* Has relevant doc changes.
+* Has relevant unit tests.
+
+1. Merge using "squash and merge".
+2. Make sure to edit the commit message to keep all the useful information
+   nice and clean.
+3. Make sure the commit title is clear and contains the PR number (#123).
+
+#### New release
+
+1. Go to [releases][releases]. Click on "X commits to master since this
+   release".
+2. Make note of all the changes. Look for backward incompatible changes,
+   new features, and bug fixes.
+3. Pick the new version using the above and semver.
+4. Create a [new release][new-release].
+5. Follow the same format as [1.1.0][release-110].
+
+[issues-tracker]: https://github.com/pelletier/go-toml/issues
+[bug-report]: https://github.com/pelletier/go-toml/issues/new?template=bug_report.md
+[godoc]: https://godoc.org/github.com/pelletier/go-toml
+[readme]: ./README.md
+[fork]: https://help.github.com/articles/fork-a-repo
+[pull-request]: https://help.github.com/en/articles/creating-a-pull-request
+[releases]: https://github.com/pelletier/go-toml/releases
+[new-release]: https://github.com/pelletier/go-toml/releases/new
+[release-110]: https://github.com/pelletier/go-toml/releases/tag/v1.1.0