fix(doc): Add compilation instructions for aya ebpf example

Signed-off-by: Wouter Dullaert <wouter.dullaert@exoscale.ch>

examples/allowed-memory/README.md

@@ -0,0 +1,10 @@
+This directory contains an example eBPF program for testing memory access with
+helper functions in rbpf.
+
+To recompile the program, make sure to install all of
+[Aya's prerequisites](https://aya-rs.dev/book/start/development/),
+then run from this directory:
+
+```bash
+cargo build --release
+```