doc: add Contributing Guide

close #23

CONTRIBUTING.md

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+# Contributing to AVR8js
+
+First of all, thank you for considering contributing to AVR8js! 
+Please go over these guidelines to ensure that your contribution lands
+successfully.
+
+## How to Contribute
+
+Before starting to work on a new feature, please 
+[file an issue](https://github.com/wokwi/avr8js/issues/new)
+to discuss the implementation. 
+
+## Setting-up Your Environment
+
+The source code is written in the [TypeScript](https://www.typescriptlang.org/) language, a typed
+extension of JavaScript.
+
+In addition, we use the following tools:
+* [prettier](https://prettier.io/) to keep the code pretty
+* [eslint](https://eslint.org/) to keep the code consistent and clean
+* [editorconfig](https://editorconfig.org/) to keep indentation consistent across different editors
+* [jest](https://jestjs.io/) for the unit tests
+
+If you open this project with [Visual Studio Code](https://code.visualstudio.com/), you will be prompted
+to install the [relevant extensions](.vscode/extensions.json) for these tools.
+
+Finally, we recommend using [Wallaby.js](https://wallabyjs.com/) to run the tests automatically
+as you write the code. It should work out of the box with this repo, without any extra configuration.
+You can also run the tests manually, from the commandline (see below).
+
+## Running the Demo Project
+
+The demo project allows you to edit Arduino code, compile it, and run it in the simulator.
+It also simulates 2 LEDs connected to pins 12 and 13 (PB4 and PB5). To run it, simply execute
+
+```
+npm start
+```
+
+Then go to http://localhost:1234/ to interact with the project.
+
+The demo project is packaged using [parcel](https://parceljs.org/) and uses the 
+[Monaco Editor](https://microsoft.github.io/monaco-editor/) for the interactive
+code editor, and the [Wokwi Elements library](https://www.npmjs.com/package/@wokwi/elements)
+for displaying the LEDs.
+
+## Running The Tests
+
+Run the tests once:
+
+```
+npm test
+```
+
+Run the tests of the files you modified since last commit (watch mode):
+
+```
+npm run test:watch
+```
+
+## Reference Material
+
+The following datasheets can be useful when working on new AVR8js features
+or fixing existing code:
+
+* [ATmega48A/PA/88A/PA/168A/PA/328/P Datasheet](http://ww1.microchip.com/downloads/en/DeviceDoc/ATmega48A-PA-88A-PA-168A-PA-328-P-DS-DS40002061A.pdf)
+* [The AVR Istruction Set Manual](http://ww1.microchip.com/downloads/en/devicedoc/atmel-0856-avr-instruction-set-manual.pdf)
+* [ ATmega640/V-1280/V-1281/V-2560/V-2561/V Datasheet](https://ww1.microchip.com/downloads/en/devicedoc/atmel-2549-8-bit-avr-microcontroller-atmega640-1280-1281-2560-2561_datasheet.pdf)
+
+## Coding Guidelines
+
+Please make sure to follow these guidelines when contributing code:
+
+1. You include a relevant test case. Ideally the test case would fail before
+   your code changes, and pass after implementing the change.
+2. Your commit messages should follow the [conventional commits 
+   standard](https://www.conventionalcommits.org/), e.g.:
+   `feat(instruction): implement EICALL, EIJMP`
+3. The contributed code has to be compatible with the MIT license. If your
+   work incoporates some third-party code, please make sure that their 
+   license is compatible and that you credit appropriately.
+
+Thank you!

README.md

@@ -26,6 +26,8 @@ Run the tests of the files you modified since last commit (watch mode):
 npm run test:watch
 ```
 
+For more information, please check the [Contributing Guide](CONTRIBUTING.md).
+
 ## License
 
-Copyright (C) 2019, Uri Shaked. The code is released under the terms of the MIT license.
+Copyright (C) 2019, 2020 Uri Shaked. The code is released under the terms of the MIT license.