[skip changelog] Revamp the README (#603)

* fix builds when merging PRs

* move contributing guidelines into docs

* restore markdown format for the README

* move away documentation from the README, use dedicated pages

* add logo

Author: Massimiliano Pippi

.github/workflows/docs.yaml

@@ -2,6 +2,10 @@ name: docs
 
 on:
   pull_request:
+    types:
+      - opened
+      - synchronize
+      - closed
     paths:
       - 'docs/**'
       - 'docsgen/**'

README.md

@@ -0,0 +1,45 @@
+# arduino-cli
+
+![cli-logo](./docs/img/featured-CLI.png)
+
+Arduino CLI is an all-in-one solution that provides builder, boards/library manager,
+uploader, discovery and many other tools needed to use any Arduino compatible
+board and platforms.
+
+[![tests-badge]](https://github.com/Arduino/arduino-cli/actions?workflow=test)
+[![nightly-badge]](https://github.com/Arduino/arduino-cli/actions?workflow=nightly)
+[![codecov-badge]](https://codecov.io/gh/arduino/arduino-cli)
+
+> **Note:** this software is currently under active development: anything can change
+  at any time, API and UI must be considered unstable until we release version 1.0.0.
+
+## Docs
+
+For guidance on installation and development, see the [User documentation].
+
+## Quickstart
+
+1. [Install] the Arduino CLI
+2. Follow the [Getting Started] guide to check out what the CLI can do
+3. Browse the [Commands reference] to see all the available commands
+4. Should you have an issue, read the [FAQ] page
+
+## How to contribute
+
+Contributions are welcome!
+
+Please read the document [How to contribute] which will show you how to build
+the source code, run the tests, and contribute your changes to the project.
+
+:sparkles: Thanks to all our [contributors]! :sparkles:
+
+
+[tests-badge]: https://github.com/Arduino/arduino-cli/workflows/test/badge.svg
+[nightly-badge]: https://github.com/Arduino/arduino-cli/workflows/nightly/badge.svg
+[codecov-badge]: https://codecov.io/gh/arduino/arduino-cli/branch/master/graph/badge.svg
+[User documentation]: https://arduino.github.io/arduino-cli/
+[Getting Started]: https://arduino.github.io/arduino-cli/getting-started/
+[Commands reference]: https://arduino.github.io/arduino-cli/commands/
+[FAQ]: https://arduino.github.io/arduino-cli/FAQ/
+[How to contribute]: https://arduino.github.io/arduino-cli/CONTRIBUTING/
+[contributors]: https://github.com/arduino/arduino-cli/graphs/contributors