Add a README.md file.

.prettierrc

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     "printWidth": 120,
     "semi": true,
     "singleQuote": true,
-    "trailingComma": "none"
+    "trailingComma": "none",
+    "overrides": [{ "files": "*.md", "options": { "tabWidth": 1 } }]
 }

README.md

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+# PG CodeMirror 6 Editor
+
+This package implements a CodeMirror 6 editor that is primarily intended for editing PG problem files for the WeBWorK
+Online Homework Delivery System. However, it also supports editing Perl, HTML, XML, Mojolicious HTML templates, and
+Mojolicious raw text templates since those are needed by webwork2.
+
+The `dist/pg-codemirror-editor.js` can be used via a script tag and sets the global `PGCodeMirrorEditor` object. The
+`PGCodeMirrorEditor.View` class is the only key in the object. It can be used as follows
+
+```Javascript
+const pgEditor = new PGCodeMirrorEditor.View(codeMirrorElt, { source: sourceCode });
+```
+
+## `PGCodeMirrorEditor.View` class
+
+The following methods and attributes for the `PGCodeMirrorEditor.View` class are available. Note that although the theme
+and keymap can be set via initialization options or provided methods, usually you do not want to do this. The theme and
+keymap can be changed by the user via the user interface provided in the editor, and the user's choices are saved in
+local storage in the browser.
+
+### Constructor
+
+The syntax for usage of the `PGCodeMirrorEditor.View` is
+
+```Javascript
+PGCodeMirrorEditor.View(
+    element: HTMLElement,
+    options?: {
+        language?: string;
+        source?: string;
+        theme?: string;
+        keyMap?: string;
+});
+```
+
+The options are described below.
+
+- `language`: One of 'pg', 'perl', 'html', 'xml', 'mt-html', or 'mt-text'. (Defaults to 'pg')
+- `source`: A string containing the source code to edit. (Defaults to the empty string)
+- `theme`: A string naming one of the following themes. (Defaults to 'Default')
+  - 'Default': A modified version of the CodeMirror 6 default theme defined in `src/light-theme.ts`.
+  - 'Amy': From the [thememirror](https://thememirror.net/) package.
+  - 'Ayu Light': From the [thememirror](https://thememirror.net/) package.
+  - 'Barf': From the [thememirror](https://thememirror.net/) package.
+  - 'Basic Dark': From the [cm6-theme-basic-dark](https://github.com/craftzdog/cm6-themes) package.
+  - 'Basic Light': From the [cm6-theme-basic-light](https://github.com/craftzdog/cm6-themes) package.
+  - 'Bespin': From the [thememirror](https://thememirror.net/) package.
+  - 'Birds of Paradise': From the [thememirror](https://thememirror.net/) package.
+  - 'Boys and Girls': From the [thememirror](https://thememirror.net/) package.
+  - 'Clouds': From the [thememirror](https://thememirror.net/) package.
+  - 'Cobalt': From the [thememirror](https://thememirror.net/) package.
+  - 'Cool Glow': From the [thememirror](https://thememirror.net/) package.
+  - 'Dracula': From the [thememirror](https://thememirror.net/) package.
+  - 'Espresso': From the [thememirror](https://thememirror.net/) package.
+  - 'Gruvbox Dark': From the [cm6-theme-gruvbox-dark](https://github.com/craftzdog/cm6-themes) package.
+  - 'Gruvbox Light': From the [cm6-theme-gruvbox-light](https://github.com/craftzdog/cm6-themes) package.
+  - 'Material Dark': From the [cm6-theme-material-dark](https://github.com/craftzdog/cm6-themes) package.
+  - 'Noctis Lilac': From the [thememirror](https://thememirror.net/) package.
+  - 'Nord': From the [cm6-theme-nord](https://github.com/craftzdog/cm6-themes) package.
+  - 'One Dark': From the [@codemirror/theme-one-dark](https://github.com/codemirror/theme-one-dark) package.
+  - 'Rose Pine Dawn': From the [thememirror](https://thememirror.net/) package.
+  - 'Smoothy': From the [thememirror](https://thememirror.net/) package.
+  - 'Solarized Dark': From the [cm6-theme-solarized-dark](https://github.com/craftzdog/cm6-themes) package.
+  - 'Solarized Light': From the [cm6-theme-solarized-light](https://github.com/craftzdog/cm6-themes) package.
+  - 'Solarized Light 2': From the [thememirror](https://thememirror.net/) package.
+  - 'Tomorrow': From the [thememirror](https://thememirror.net/) package.
+- `keymap`: One of 'Default', 'Emacs', or 'Vim'. (Defaults to 'Default')
+
+### `source: string`
+
+Set the source code to be edited with the `source` getter, and get the current source with the `source` setter. Note
+that setting the source will reset the editor state (including undo and redo history).
+
+### `setLanguage(languageName: string): Promise<void>`
+
+Set the language for the editor after initialization. The `languageName` must be one of the languages listed for the
+constructor `language` option. Note that this method is `async`.
+
+### `setTheme(themeName: string): Promise<void>`
+
+Set the theme for the editor after initialization. The `themeName` must be one of the themes listed for the constructor
+`theme` option. Note that this method is `async`.
+
+### `setKeyMap(keyMapName: string): Promise<void>`
+
+Set the keymap for the editor after initialization. The `keyMapName` must be one of the keymaps listed for the
+constructor `keymap` option. Note that this method is `async`.
+
+### `refresh(key: string): void`
+
+Schedule a layout measurement for the editor. Provide a string `key` that identifies the call. When multiple requests
+with the same key are scheduled, only the last one will actually be run.
+
+## Development Testing
+
+Executing `npm run serve` will make an html test page available at `http://localhost:8080`.