docs: add document about local development

docs/local-development.md

@@ -0,0 +1,42 @@
+## Prerequesites
+
+* [Node.js](https://nodejs.org/) v12+
+* [npm](https://www.npmjs.com/package/npm)
+* [npx](https://www.npmjs.com/package/npx)
+
+## Build
+
+Building your clone of the repository is easy:
+
+1. run `npm install`.
+1. run `npx grunt build` (see [`Gruntfile.js`][gruntfile]). This command:
+
+    1. Cleans the repo.
+    1. Checks code quality ([JSHint](https://jshint.com/) and [ESLint](https://eslint.org/)).
+    1. Runs tests.
+    1. Creates the [distributable][sd-dist] and [minified][sd-min] files in the [`dist`][dist-folder] folder.
+
+## Test
+
+A suite of tests is available which require Node.js. Once Node is installed, run the following command from
+the project root to install the dependencies:
+
+```
+npm install
+```
+
+Once installed, run tests from the project root:
+
+```
+npm test
+```
+
+You can easily add new tests:
+
+1. Create a markdown file (ending in `.md`) that contains the markdown to test.
+1. Create a `.html` file with the same name as the markdown one from the previous step. This `html` file will automatically be tested when the tests are executed with `mocha`.
+
+[gruntfile]: https://github.com/showdownjs/showdown/blob/master/Gruntfile.js
+[sd-dist]: https://github.com/showdownjs/showdown/blob/master/dist/showdown.js
+[sd-min]: https://github.com/showdownjs/showdown/blob/master/dist/showdown.min.js
+[dist-folder]: https://github.com/showdownjs/showdown/tree/master/dist

mkdocs.yml

@@ -48,4 +48,5 @@ nav:
     - Overview: extensions.md
     - Create an extension: create-extension.md
     - List of known extensions: extensions-list.md 
-  - Tutorials: tutorials/index.md
+  - Tutorials: tutorials/index.md
+  - Local development: local-development.md