Add contributing file (#1305)

* Add CONTRIBUTING.md

* Update README.md

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+# Issues and Features
+
+Issues reported on [GitHub](https://github.com/FooSoft/yomichan/issues) should include information about:
+
+* What the problem, question, or request is.
+* What browser is being used.
+* What version of Yomichan is being used.
+* If applicable, an export of the settings file.
+
+# Development
+
+Contributions are welcome from any developers who would like to help out.
+Below are a few guidelines to ensure contributions have a good level of quality and consistency:
+
+* Open GitHub issues to discuss large features before writing code.
+* Follow the [conventions and style](#style) of the existing code.
+* Test changes using the continuous integration tests included in the repository.
+* Write clean, modern ES6 code (`const`/`let`, `async`/`await`, arrow functions, etc.)
+* Large pull requests without a clear scope will not be merged.
+* Incomplete or non-standalone features will not be merged.
+
+## Setup
+
+Yomichan uses [Node.js](https://nodejs.org/) and [npm](https://www.npmjs.com/) tools for building and testing.
+After installing these Node.js, the development environment can be set up by running `npm ci`.
+
+## Testing
+
+Unit tests, integration tests, and various other tests can be executed by running `npm test`.
+Other individual tests can be looked up in the [package.json](package.json) file, and the source for specific tests
+can be found in the [test](test) directory
+
+## Building
+
+By default, the development repository is configured for Chrome, and the [ext](ext) directory can be directly
+loaded as an unpacked extension by Chrome. This way, development does not require any additional build steps,
+and most changes will be automatically updated by the browser. Depending on what files were changed,
+the extension may sometimes need to be reloaded before the changes take effect.
+
+
+There are two scripts to build the extension to a packaged file for various build targets:
+* [build.bat](build.bat) on Windows
+* [build.sh](build.sh) on Linux
+
+Both of these files are convenience scripts which invoke <code>node [dev/build.js](dev/build.js)</code>.
+The build script can produce several different build files based on manifest configurations defined in
+[manifest-variants.json](dev/data/manifest-variants.json).
+Several command line arguments are available for these scripts:
+
+* `[target]` - Builds a specific target.
+* `--all` - Builds all targets specified in [manifest-variants.json](dev/data/manifest-variants.json).
+* `--default` - Restores the default manifest file.
+* `--manifest <target>` - Overwrites [ext/manifest.json](ext/manifest.json) with the manifest variant for the specified build target.
+* `--dry-run` - Runs the full build process (excluding zip building), checking that the configuration is valid.
+* `--dry-run-build-zip` - If `--dry-run` is also specified, zip building will also be performed in memory; no files are created.
+
+If no arguments are specified, the command is equivalent to `build.bat --all`.
+
+### Build Tools
+
+The build process can use the [7-zip](https://www.7-zip.org/) archiving tool to create the packed zip builds
+if the 7-zip executable (either `7z` or `7za`) is found in the `PATH` environment variable.
+Otherwise, the [JSZip](https://stuk.github.io/jszip/) API is used to generate the files.
+7-zip typically provides better compression than JSZip, but the files are otherwise equivalent.
+
+## Manifest
+
+Manifest variants for different build targets are specified in [manifest-variants.json](dev/data/manifest-variants.json).
+This file is used to overwrite the [manfiest.json](ext/manifest.json) file included in the extension.
+By default, this manifest should be the default `chrome` manifest, and changes to [manfiest.json](ext/manifest.json) should not be committed
+unless there is a corresponding change in [manifest-variants.json](dev/data/manifest-variants.json).
+There is a continuous integration test which validates this, and the default manifest can be restored by running
+`build.bat --default`.
+
+## Style
+
+Linting rules are defined for a few types of files, and validation is performed as part of the standard tests
+run by `npm test` and the continuous integration process.
+
+* [.eslintrc.json](.eslintrc.json) rules are used for JavaScript files.
+* [.stylelintrc.json](.stylelintrc.json) rules are used for CSS files.