Add contribution guide #527 (#528)

Co-authored-by: Florent <florent.fayolle@beta.gouv.fr>

.github/CONTRIBUTING.md

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+# Welcome to the contribution guide for Grist!
+
+You are eager to contribute to Grist? That's awesome! See below some contributions you can make:
+- [translate](/documentation/translate.md)
+- [write tutorials and user documentation](https://github.com/gristlabs/grist-help)
+- [develop](/documentation/develop.md)
+- [report issues or suggest enhancement](https://github.com/gristlabs/grist-core/issues/new)
+

documentation/develop.md

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+# Development
+
+Please as a first start, tell the community about your intent to develop a feature or fix a bug. Search for the associated issue if it exists or open one with steps to reproduce (for bugs) or a [user story](https://en.wikipedia.org/wiki/User_story#Principle) (for features).
+
+## Setup
+
+### Prerequisites
+
+To setup your environment, you would need to install the following dependencies:
+ - git
+ - [nvm](https://github.com/nvm-sh/nvm/blob/master/README.md) (recommended) or nodejs installed on your system
+ - Chromium to run the end-to-end tests
+ - Python (preferably Python 3.9) and virtualenv
+    - :warning: As of 2023-06-06, Python 3.11 is not supported due to the version of the [wrapt dependency](https://github.com/GrahamDumpleton/wrapt/issues/196)
+
+### Clone the repository
+
+```bash
+$ git clone https://github.com/gristlabs/grist-core
+```
+
+And then, enter the grist-core root directory:
+
+```bash
+$ cd grist-core/
+```
+
+### Setup nodejs
+
+#### Using nvm (recommanded)
+
+You need to install the supported nodejs version as well as yarn. To do so, in the grist-core root directory, run the following command to install nodejs via nvm:
+
+```bash
+$ nvm install
+```
+
+Now check that node is installed in the version specified in the `.nvmrc` file:
+
+```bash
+$ node --version
+```
+
+Then install yarn (the `-g` flag here means that yarn will be available globally):
+```bash
+$ npm install -g yarn
+```
+
+Now each time you want to load nodejs and yarn in your environment, just run the following command at grist-core root directory:
+
+```bash
+$ nvm use
+```
+
+#### Using nodejs
+
+You can also use nodejs installed in your system. To prevent incompatibilities, ensure that the `node --version` command reports a version equal or greater to the one in `.nvmrc`.
+
+### Install the python packages
+
+Be sure to have Python and virtualenv installed. On debian-based Linux distributions, you can simply run the following command as root:
+
+```bash
+# apt install python3.9 python3.9-venv
+```
+
+### Install the project dependencies and build
+
+First install the nodejs dependencies:
+
+```bash
+$ yarn install
+```
+
+Then prepare the virtual environment with all the python dependencies:
+
+```bash
+$ yarn install:python
+```
+
+Finally run this to do an initial build:
+
+```bash
+$ yarn run build:prod
+```
+
+## Start the server in development mode
+
+Just run the following command:
+```bash
+$ yarn start
+```
+
+Each time you change something, just reload the webpage in your browser.
+
+Happy coding!
+
+### Pick an issue
+
+Lost on what you can do to help? If you are new to Grist, you may just pick one of the issues labelled `good first issue`:
+
+https://github.com/gristlabs/grist-core/labels/good%20first%20issue
+
+## Run tests
+
+You may run the tests using one of these commands:
+ - `yarn test` to run all the tests
+ - `yarn test:smoke` to run the minimal test checking Grist can open, create and edit a document
+ - `yarn test:nbrowser` to run the end-to-end tests
+ - `yarn test:client` to run the tests for the client libraries
+ - `yarn test:common` to run the tests for the common libraries shared between the client and the server
+ - `yarn test:server` to run the backend tests
+ - `yarn test:docker` to run some end-to-end tests under docker
+ - `yarn test:python` to run the data engine tests
+
+## Develop widgets
+
+Check out this repository: https://github.com/gristlabs/grist-widget#readme
+
+## Documentation
+
+Some documentation to help you starting developing:
+ - [Grainjs](https://github.com/gristlabs/grainjs/) (The library used to build the DOM)
+ - [The user support documentation](https://support.getgrist.com/)