README template for stm32yyxx-hal crates

[marcoieni]

At the moment every hal crate (i.e. l4xx, f4xx and so on) has its own README format.

we should define a standard template.

For example:

# stm32yyxx-hal

[![Crates.io](https://img.shields.io/crates/v/stm32yyxx-hal.svg)](https://crates.io/crates/stm32yyxx-hal)
[![Docs](https://docs.rs/stm32yyxx-hal/badge.svg)](https://docs.rs/stm32yyxx-hal)

## About

- Minimum rustc version 1.xx

## Setting up your project

Check if the BSP for your board exists in the
[stm32-rs](https://github.com/stm32-rs) page.
If it exists, the `stm32yyxx-hal` crate should be already included, so you can
use the bsp as BSP for your project.

Otherwise, create a new Rust project as you usually do with `cargo init`. The
"hello world" of embedded development is usually to blink a LED. The code to do
so is available in [examples/delay-blinky.rs](examples/delay-blinky.rs).
Copy that file to the `main.rs` of your project.

You also need to add some dependencies to your `Cargo.toml`:

toml
[dependencies]
embedded-hal = "0.2"
nb = "0.1.2"
cortex-m = "0.6"
cortex-m-rt = "0.6"
# Panic behaviour, see https://crates.io/keywords/panic-impl for alternatives
panic-halt = "0.2"

[dependencies.stm32yyxx-hal]
version = "0.8"
features = ["rt", "stm32yyxx"] # replace the model of your microcontroller here


## License

I will not go more in detail because defining the template in this issue is not very convenient. Let's just choose the path of this file I will make a pull request where we talk about it.
I propose: hal_template/README.md
In the future in this directory I would like to place also template for issues and pull requests, contributing guidelines, github actions and so on.

EDIT: we could also create a template repository instead of a subdirectory of this repo.