Add CONTRIBUTING.md (fixes #127).

CONTRIBUTING.MD

@@ -0,0 +1,47 @@
+# Contributing
+
+Interested in contributing? Great! Here are some suggestions to make it a good experience:
+
+Start by [opening an issue](issues), whether to identify a problem or outline a change.
+That issue should be used to discuss the situation and agree on a plan of action before writing code or sending a pull request.
+Maybe the problem isn't really a problem, or maybe there are more things to consider.
+If so, it's best to realize that before spending time and effort writing code that may not get used.
+
+Match the coding style of the files you edit.
+Although everyone has their own preferences and opinions, a pull request is not the right forum to debate them.
+
+Do not add new [`dependencies` to `package.json`](https://docs.npmjs.com/files/package.json#dependencies).
+The excellent Markdown parser [markdown-it](https://www.npmjs.com/package/markdown-it) is this project's one and only dependency.
+
+If developing a new rule, start by creating a [custom rule](doc/CustomRules.md) in its own project.
+Once written, published, and tested in real world scenarios, open an issue to consider adding it to this project.
+For rule ideas, see [issues tagged with the `new rule` label](labels/new%20rule).
+
+Add tests for all new/changed functionality.
+Test positive and negative scenarios.
+Try to break the new code now, or else it will get broken later.
+
+Run tests before sending a pull request via `npm test` in the [usual manner](https://docs.npmjs.com/misc/scripts).
+Tests should all pass on all platforms.
+The test runner is [nodeunit](https://www.npmjs.com/package/nodeunit) and test cases are located in `test/markdownlint-test.js`.
+When running tests, `test/*.md` files are enumerated, linted, and fail if any violations are missing a corresponding `{MD###}` marker in the test file.
+For example, the line `### Heading {MD001}` is expected to trigger the rule `MD001`.
+For cases where the marker text can not be present on the same line, the syntax `{MD###:#}` can be used to include a line number.
+If `some-test.md` needs custom configuration, a `some-test.json` is used to provide a custom `options.config` for that scenario.
+
+Lint before sending a pull request by running `npm run lint`.
+There should be no issues.
+
+Run code coverage before sending a pull request with `npm run test-cover`.
+Coverage should remain at 100%.
+
+Pull requests should contain a single commit.
+If necessary, squash multiple commits before creating the pull request and when making changes.
+(See [Git Tools - Rewriting History](https://git-scm.com/book/en/v2/Git-Tools-Rewriting-History) for details.)
+
+Open pull requests against the `next` branch.
+That's where the latest changes are staged for the next release.
+Include the text "(fixes #??)" at the end of the commit message so the pull request will be associated with the relevant issue.
+Once accepted, the tag `fixed in next` will be added to the issue.
+When the commit is merged to `master` branch during the release process, the issue will be closed automatically.
+(See [Closing issues using keywords](https://help.github.com/articles/closing-issues-using-keywords/) for details.)

README.md

@@ -679,6 +679,10 @@ For ideas how to integrate `markdownlint` into your workflow, refer to the follo
 * [TestCafe](https://devexpress.github.io/testcafe/) ([Search repository](https://github.com/DevExpress/testcafe/search?q=markdownlint))
 * [webpack](https://webpack.js.org/) ([Search repository](https://github.com/webpack/webpack.js.org/search?q=markdownlint))
 
+## Contributing
+
+See [CONTRIBUTING.md](CONTRIBUTING.md) for more information.
+
 ## History
 
 * 0.0.1 - Initial release, includes tests MD001-MD032.

test/markdownlint-test.js

@@ -94,7 +94,10 @@ fs.readdirSync("./test").forEach(function forFile(file) {
 module.exports.projectFiles = function projectFiles(test) {
   test.expect(2);
   const options = {
-    "files": [ "README.md" ],
+    "files": [
+      "README.md",
+      "CONTRIBUTING.md"
+    ],
     "noInlineConfig": true,
     "config": {
       "MD013": { "line_length": 150 },
@@ -103,7 +106,10 @@ module.exports.projectFiles = function projectFiles(test) {
   };
   markdownlint(options, function callback(err, actual) {
     test.ifError(err);
-    const expected = { "README.md": [] };
+    const expected = {
+      "README.md": [],
+      "CONTRIBUTING.md": []
+    };
     test.deepEqual(actual, expected, "Issue(s) with project files.");
     test.done();
   });
@@ -930,6 +936,7 @@ module.exports.readmeHeadings = function readmeHeadings(test) {
           "## Usage",
           "## Browser",
           "## Examples",
+          "## Contributing",
           "## History"
         ]
       }