doc: add links to the manual (#705)

Author: molpopgen

README.md

@@ -40,6 +40,9 @@ cargo test --all-features
 cargo doc --all-features --open
 ```
 
+API documentation for the latest release is [here](https://docs.rs/tskit).
+A manual is [here](https://tskit-dev.github.io/tskit-rust).
+
 ### Calculating code coverage
 
 First, install `tarpaulin`:

src/lib.rs

@@ -68,6 +68,10 @@
 //!
 //! * A lot of wrappers to the C functions.
 //! * Tree sequence statistics!
+//!
+//! # Manual
+//!
+//! A manual is [here](https://tskit-dev.github.io/tskit-rust).
 
 #![allow(non_upper_case_globals)]
 #![allow(non_camel_case_types)]