doc: add migration guide to readme

README.md

@@ -21,9 +21,9 @@ which demonstrate reading files of arbitrary size into a memory-mapped buffer
 and passing them through the different chunker implementations.
 
 ```shell
-$ cargo run --example v2016 -- --size 16384 test/fixtures/SekienAkashita.jpg
+$ cargo run --example v2020 -- --size 16384 test/fixtures/SekienAkashita.jpg
     Finished dev [unoptimized + debuginfo] target(s) in 0.03s
-     Running `target/debug/examples/v2016 --size 16384 test/fixtures/SekienAkashita.jpg`
+     Running `target/debug/examples/v2020 --size 16384 test/fixtures/SekienAkashita.jpg`
 hash=17968276318003433923 offset=0 size=21325
 hash=4098594969649699419 offset=21325 size=17140
 hash=15733367461443853673 offset=38465 size=28084
@@ -38,7 +38,7 @@ code snippet is an example:
 let read_result = fs::read("test/fixtures/SekienAkashita.jpg");
 assert!(read_result.is_ok());
 let contents = read_result.unwrap();
-let chunker = fastcdc::v2016::FastCDC::new(&contents, 16384, 32768, 65536);
+let chunker = fastcdc::v2020::FastCDC::new(&contents, 16384, 32768, 65536);
 let results: Vec<Chunk> = chunker.collect();
 assert_eq!(results.len(), 2);
 assert_eq!(results[0].offset, 0);
@@ -47,21 +47,44 @@ assert_eq!(results[1].offset, 66549);
 assert_eq!(results[1].length, 42917);
 ```
 
+## Migration from pre-3.0
+
+If you were using a release of this crate from before the 3.0 release, you will need to make a small adjustment to continue using the same implemetation as before.
+
+Before the 3.0 release:
+
+```rust
+use fastcdc::ronomon as fastcdc;
+use std::fs;
+let contents = fs::read("test/fixtures/SekienAkashita.jpg").unwrap();
+let chunker = fastcdc::FastCDC::new(&contents, 8192, 16384, 32768);
+```
+
+After the 3.0 release:
+
+```rust
+use std::fs;
+let contents = fs::read("test/fixtures/SekienAkashita.jpg").unwrap();
+let chunker = fastcdc::ronomon::FastCDC::new(&contents, 8192, 16384, 32768);
+```
+
+The cut points produced will be identical to previous releases as the `ronomon` implementation was never changed in that manner. Note, however, that the other implementations _will_ produce different results.
+
 ## Reference Material
 
 The original algorithm is described in [FastCDC: a Fast and Efficient Content-Defined Chunking Approach for Data Deduplication](https://www.usenix.org/system/files/conference/atc16/atc16-paper-xia.pdf), while the improved "rolling two bytes each time" version is detailed in [The Design of Fast Content-Defined Chunking for Data Deduplication Based Storage Systems](https://ieeexplore.ieee.org/document/9055082).
 
 ## Other Implementations
 
 * [jrobhoward/quickcdc](https://github.com/jrobhoward/quickcdc)
-    + Similar but slightly earlier algorithm by some of the same researchers?
+    + Similar but slightly earlier algorithm by some of the same authors?
 * [rdedup_cdc at docs.rs](https://docs.rs/crate/rdedup-cdc/0.1.0/source/src/fastcdc.rs)
     + Alternative implementation in Rust.
 * [ronomon/deduplication](https://github.com/ronomon/deduplication)
     + C++ and JavaScript implementation of a variation of FastCDC.
 * [titusz/fastcdc-py](https://github.com/titusz/fastcdc-py)
     + Pure Python port of FastCDC. Compatible with this implementation.
-* [wxiacode/destor](https://github.com/wxiacode/destor/blob/master/src/chunking)
+* [wxiacode/FastCDC-c](https://github.com/wxiacode/FastCDC-c)
     + Canonical algorithm in C with gear table generation and mask values.
 * [wxiacode/restic-FastCDC](https://github.com/wxiacode/restic-FastCDC)
     + Alternative implementation in Go with additional mask values.

TODO.org

@@ -1,6 +1,5 @@
 * Action Items
 ** TODO Rewrite
-*** TODO doc: add migration guide to =README.md=
 *** TODO incorporate some form of streaming support based on =Read=
 **** c.f. https://gitlab.com/asuran-rs/asuran/ (asuran-chunker, uses =fastcdc= with =Read=)
 **** basically just allocate a buffer 2*max and fill it as needed