Update CONTRIBUTING.md

Author: norvig

CONTRIBUTING.md

@@ -9,6 +9,15 @@ Thanks for considering contributing to `aima-python`! Here is some of the work t
 - Replace old Lisp-based idioms with proper Python idioms. For example, we have many functions that were taken directly from Common Lisp, such as the `every` function: `every(callable, items)` returns true if every element of `items` is callable. This is good Lisp style, but good Python style would be to use `all` and a generator expression: `all(callable(f) for f in items)`. Eventually, fix all calls to these legacy Lisp functions and then remove the functions.
 - Add more tests in `_test.py` files. Strive for terseness; it is ok to group multiple asserts into one `def test_something():` function. Move most tests to `_test.py`, but it is fine to have a single `doctest` example in the docstring of a function in the `.py` file, if the purpose of the doctest is to explain how to use the function, rather than test the implementation.
 
+## New and Improved Algorithms
+
+- Implement functions that were in the third edition of the book but were not yet implemented in the code. Check the [list of pseudocode algorithms (pdf)](https://github.com/aimacode/pseudocode/blob/master/algorithms.pdf) to see what's missing.
+- As we finish chapters for the new fourth edition, we will share the new pseudocode in the [`aima-pseudocode`](https://github.com/aimacode/aima-pseudocode) repository, and describe what changes are necessary.
+We hope to have a `algorithm-name.md` file for each algorithm, eventually; it would be great if contributors could add some for the existing algorithms.
+- Give examples of how to use the code in the `.ipynb` file.
+
+We still support a legacy branch, `aima3python2` (for the third edition of the textbook and for Python 2 code). 
+
 # Style Guide
 
 There are a few style rules that are unique to this project: