docs: add a README

How to install and run the snap, and how to embed the library: one
RunOptions example covers a basic machine. The logo lives under
.github/img.

README.md

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+<p align="center">
+  <img src=".github/img/logo.svg" width="150" alt="écrou logo">
+</p>
+<h1 align="center">ecrou</h1>
+<p align="center">a small microVM runtime based on <a href="https://github.com/libkrun/libkrun">libkrun</a></p>
+
+## Try ecrou in the terminal
+
+### Installation
+
+ecrou is available as a strictly confined snap:
+```
+sudo snap install ecrou
+sudo snap connect ecrou:kvm
+```
+
+### Usage
+
+```
+$ ecrou run -it ubuntu
+root@ecrou-vm-m2glhl:/# 
+```
+
+## Embed ecrou into your project
+
+ecrou is both a library and a tiny CLI frontend around it. The library is embeddable into any project, allowing to use microVMs easily.
+
+### Rust
+
+ecrou is a Rust library, which can just be called:
+```rust
+use ecrou::vm::RunOptions;
+use ecrou::NoProgress;
+
+fn main() -> ecrou::Result<std::process::ExitCode> {
+    let code = ecrou::vm::run(
+        &RunOptions {
+            image: "ubuntu:resolute".into(),
+            command: vec!["uname".into(), "-a".into()],
+            ..RunOptions::default()
+        },
+        &NoProgress,
+    )?;
+    Ok(std::process::ExitCode::from(code.clamp(0, 255) as u8))
+}
+```
+