add plugin authoring guide, close #399

PLUGINS.md

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+# Leaflet Plugin Authoring Guide
+
+One of the greatest things about Leaflet is its powerful plugin ecosystem. 
+The [Leaflet plugins page](http://leafletjs.com/plugins.html) lists dozens of awesome plugins, and more are being added every week.
+
+This guide lists a number of best practices for publishing a perfect Leaflet plugin that meets the quality standards of Leaflet itself.
+
+1. [Presentation](#presentation)
+	- [Repository](#repository)
+	- [Demo](#demo)
+	- [Readme](#readme)
+	- [License](#license)
+2. [Code](#code)
+	- [File Structure](#file-structure)
+	- [Code Conventions](#code-conventions)
+	- [API](#api)
+
+## Presentation
+
+### Repository
+
+The best place to put your Leaflet plugin to is a separate [GitHub](http://github.com) repository. 
+If you create a collection of plugins for different uses, 
+don't put them in one repo — 
+it's usually easier to work with small, self-contained plugins.
+
+### Demo
+
+The most essential thing to do when publishing a plugin is putting up a demo that showcases what the plugin does — 
+it's usually the first thing people will look for.
+
+The easiest way to put up a demo is using [GitHub Pages](http://pages.github.com/). 
+A good [starting point](https://help.github.com/articles/creating-project-pages-manually) is creating a `gh-pages` branch in your repo and adding an `index.html` page to it  — 
+after pushing, it'll be published as `http://<user>.github.io/<repo>`.
+
+### Readme
+
+The next thing you need to have is a good descriptive `README.md` in the root of the repo (or a link to a website with a similar content).
+At the least, it should contain the following items:
+
+- plugin title
+- simple, concise description
+- requirements
+	- Leaflet version
+	- other external dependencies (if present)
+	- browser / device compatibility
+- links to demos
+- instructions for including the plugin
+- simple usage code example
+- API reference (methods, options, events)
+
+### License
+
+Every good open source repository should have a license specified. 
+If you don't know what open source license to choose for your code, 
+[MIT License](http://opensource.org/licenses/MIT) and [BSD 2-Clause License](http://opensource.org/licenses/BSD-2-Clause) are both good choices. 
+You can either put it in the repo as a `LICENSE` file or just link to the license from the Readme.
+
+## Code
+
+### File Structure
+
+Keep the file structure clean and simple, 
+don't pile up lots of files in one place  &mdash; 
+make it easy for a new person to find their way in your repo.
+
+A barebones repo for a simple plugin would like this:
+
+```
+my-plugin.js
+README.md
+```
+
+An example of a file structure for a more sophisticated plugin:
+
+```
+/src        JS source files
+/dist       minified plugin JS, CSS, images
+/spec       test files
+/examples   HTML examples of plugin usage
+README.md
+LICENSE
+package.json
+```
+
+### Code Conventions
+
+Everyone's tastes are different, but it's important to be consistent with whatever conventions you choose for your plugin.
+
+For a good starting point, check out [Airbnb JavaScript Guide](https://github.com/airbnb/javascript). 
+Leaflet follows pretty much the same conventions 
+except for using smart tabs (hard tabs for indentation, spaces for alignment) 
+and putting space after `function` keyword.
+
+### Plugin API
+
+Never expose global variables with your plugin.  
+If you have a new class, put it directly in the `L` namespace (`L.MyPlugin`).  
+If you inherit one of the existing classes, put it as its property (`L.TileLayer.Banana`).
+If you want to add new methods to existing Leaflet classes, you can do it like this: `L.Marker.include({myPlugin: …})`.
+
+Function, method and property names should be in `camelCase`.  
+Class names should be in `CapitalizedCamelCase`.
+
+If you have a lot of arguments in your function, consider accepting an options object instead:
+
+```
+marker.myPlugin(arg1, {firstOption: 'foo', secondOption: 2}); // good
+marker.myPlugin(arg1, option1, option2, option3, option4);    // bad
+
+```