tor-basic-utils: copy the README into lib.rs

The doc include rune does not work with our MSRV; it needs 1.54.

The alternative would be some kind of cfg() but that would
  - not provide the crate-level doc on Rust 1.53
  - involve the use of cfg_attr

Instead, just do it the old way.

crates/tor-basic-utils/src/lib.rs

-#![doc = include_str!("../README.md")]
+//! `tor-basic-utils`: Utilities (low-level) for Tor
+//!
+//! Miscellaneous utilities for `tor-*` and `arti-*`.
+//!
+//! This crate lives at the *bottom* of the Tor crate stack.
+//! So it contains only utilities which have no `tor-*` (or `arti-*`) dependencies.
+//!
+//! There is no particular theme.
+//! More substantial sets of functionality with particular themes
+//! are to be found in other `tor-*` crates.
+
 #![deny(missing_docs)]
 #![warn(noop_method_call)]
 #![deny(unreachable_pub)]