added a small dev note about c vs. lambda: point to release v2

README.md

@@ -2,7 +2,7 @@
 [![CodeQL](https://github.com/jermp/pthash/actions/workflows/codeql.yml/badge.svg)](https://github.com/jermp/pthash/actions/workflows/codeql.yml)
 
 PTHash / PHOBIC
-------
+---------------
 
 PTHash is a C++ library implementing fast and compact minimal perfect hash functions as described in the following research papers:
 
@@ -12,6 +12,14 @@ PTHash is a C++ library implementing fast and compact minimal perfect hash funct
 
 **Please, cite these papers if you use PTHash or PHOBIC.**
 
+### Development note
+
+The description of PTHash in the SIGIR and TKDE papers uses the `c` parameter
+to control the number of buckets used during the search.
+You can get a version of the library using the `c` parameter [here](https://github.com/jermp/pthash/releases/tag/v2.0.0) (Release v2).
+The current library
+uses instead a parameter called "lambda", as described in the ESA paper.
+
 ### Features
 
 - Minimal and Non-Minimal Perfect Hash Functions