📝(doc) update docs to install published packages + contributing guide

Updated various parts of existing documentations to make a clean path
to learn how to install the library. I let the most complicated part in
the tokens's README.md file, which is referenced from the Storybook.

CONTRIBUTING.md

@@ -0,0 +1,80 @@
+# Contributing to Cunningham
+
+Want to contribute to Cunningham? We got you covered.
+
+We try to raise our code quality standards and expect contributors to follow
+the recommandations from our [handbook](https://handbook.openfun.fr/).
+
+## Development
+
+### First start
+
+Install and build packages
+
+```
+make bootstrap
+```
+
+And then run dev servers
+
+```
+make dev
+```
+
+Then, you will be able to access the demo app on [http://localhost:3200](http://localhost:3200) and the React components package's Storybook on
+[http://localhost:6006](http://localhost:6006)
+
+### Building
+
+In order to build all packages of this repo you just need to type the following command at the root of the monorepo.
+
+```
+make build
+```
+
+## Sending a pull request
+
+When submitting please follow those steps:
+
+- Make sure you followed the commit conventions from the [handbook/git](https://handbook.openfun.fr/git).
+- Make sure that you added tests covering your changes.
+- Run `make deploy` to tests, lint and build the project.
+- Create a [changeset](#adding-a-changeset) by running `yarn changeset` at the root of the repo. In any case, a bot will verify if your PR contains a changeset.
+
+### Changelog
+
+The changelog is created with [Changesets](https://github.com/changesets/changesets).
+
+#### Adding a changeset
+
+A changeset describes changes made in a branch or commit. It holds three bits of information:
+
+- What packages we need to release
+- What version we are releasing packages
+- A changelog entry for the released packages
+
+Add a changeset if you have made any changes that will require a package version bump and release:
+
+1. Run `yarn changeset`.
+2. Select the packages you want to include using ↑ and ↓ to navigate to packages, and space to select a package. Hit enter when all desired packages are selected.
+3. Select a [bump type](https://semver.org/) for each selected package.
+4. Provide a message to be written into the changelog on the next release.
+
+#### Writing a changelog message
+
+Keep the following in mind when authoring your changelog entry (final prompt after running `yarn changeset`):
+
+- Begin sentences with explicit verbs like "add", "fix", "refactor"
+- Avoid redundancy when possible
+- Use sentence case
+- Use plain language
+
+#### Out of scope for the changelog
+
+Generally, changes related to these topics can be omitted:
+
+- Doc typo fixes
+- Dev dependencies upgrades
+- Chores (infrastructure, release process…)
+
+This part of the documentation is 99% inspired by [Polaris](https://github.com/Shopify/polaris/blob/main/.github/CONTRIBUTING.md#adding-a-changeset)