Use mod vendor for vendored directory (backwards compatible)

vendor/github.com/lrstanley/girc/CONTRIBUTING.md

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+# Contributing
+
+## Issue submission
+
+   * When submitting an issue or bug report, please ensure to provide as much
+     information as possible, please ensure that you are running on the latest
+     stable version (tagged), or when using master, provide the specific commit
+     being used.
+   * Provide the minimum needed viable source to replicate the problem.
+
+## Pull requests
+
+To review what is currently being worked on, or looked into, feel free to head
+over to the [issues list](../../issues).
+
+Below are a few guidelines if you would like to contribute. Keep the code
+clean, standardized, and much of the quality should match Golang's standard
+library and common idioms.
+
+   * Always test using the latest Go version.
+   * Always use `gofmt` before committing anything.
+   * Always have proper documentation before committing.
+   * Keep the same whitespacing, documentation, and newline format as the
+     rest of the project.
+   * Only use 3rd party libraries if necessary. If only a small portion of
+     the library is needed, simply rewrite it within the library to prevent
+     useless imports.
+   * Also see [golang/go/wiki/CodeReviewComments](https://github.com/golang/go/wiki/CodeReviewComments)
+
+If you would like to assist, and the pull request is quite large and/or it has
+the potential of being a breaking change, please open an issue first so it can
+be discussed.