README: clarify license and add an example

Signed-off-by: Levente Kurusa <lkurusa@acm.org>

README.md

@@ -1,2 +1,38 @@
 # cgroups-rs
 Native Rust library for managing control groups under Linux
+
+# Example
+
+## Create a control group, and limit the pid resource
+
+``` rust
+// Acquire a handle for the V1 cgroup hierarchy.
+let hier = ::hierarchies::V1::new();
+// Create a control group named "example" in the hierarchy.
+let cg = Cgroup::new(&hier, String::from("example"), 0);
+{
+    // Get a handle to the pids controller of the control group.
+    let pids: &PidController = cg.controller_of().expect("No pids controller in V1 hierarchy!");
+    // Set the maximum amount of processes in the cgroup.
+    pids.set_pid_max(PidMax::Value(10));
+    // Check that this has had the desired effect by reading the value back from the kernel.
+    assert_eq!(pids.get_pid_max(), Some(PidMax::Value(10)));
+}
+// Once done, delete the control group (and its associated controllers).
+cg.delete();
+```
+
+# Disclaimer
+
+This crate is licensed under:
+
+- MIT License (see LICENSE-MIT); or
+- Apache 2.0 LIcense (see LICENSE-Apache-2.0),
+
+at your option.
+
+Please note that this crate is under heavy development, we will use sematic
+versioning, but during the `0.0.*` phase, no guarantees are made about
+backwards compatibility.
+
+Regardless, check back often and thanks for taking a look!