Add Docs on Unsafety to Crate Root and README

[lopopolo]

The Rc implementation of Adoptable includes some docs on the safety implications of using these APIs incorrectly. This documentation should exist prominently in the crate root and the README

cactusref/src/adoptable.rs

Lines 42 to 50 in 3870a4e

	/// # Safety
	///
	/// Modifying the object graph by removing links after calling `adopt`
	/// requires updating the bookkeeping with [`unadopt`](Adoptable::unadopt).
	/// Failure to properly remove links from the object graph may cause `Drop`
	/// to detect a cycle is not externally reachable, reulting in prematurely
	/// deallocating the entire cycle and turning all reachable [`Rc`]s into
	/// dangling pointers. `CactusRef` makes a best-effort attempt to abort the
	/// program if an access to a dead `Rc` occurs.