Add perft section to readme (#19)

Reviewed-on: #19
Co-authored-by: Josh <josh@joshuaschuett.com>
Co-committed-by: Josh <josh@joshuaschuett.com>

README.md

@@ -1,4 +1,4 @@
-# Chess Engine in C — Project Goal
+# Chess Engine in C
 
 Build a clean, fast chess engine in C that starts from a FEN string and produces fully legal moves, verified via **PERFT** for correctness and speed.
 
@@ -29,6 +29,33 @@ FEN is a single line string that describes a chess position. Format:
 - halfmove: `0`  
 - fullmove: `1`
 
+# Perft Testing A Chess Engine
+
+**Perft** (“performance test”) walks the move tree *without move evaluation* and counts nodes at certain depths in the tree. It’s a standard way to verify an engine's **move generation + make/unmake (or make-on-copy)** are correct. It's important to understand that the node counts have been generated and verified by the chess programming community, meaning that these counts are a reliable reference point for a chess engine to match against.
+
+## What perft checks
+- **Legality filtering:** no moves that leave your own king in check.
+- **Special rules:** castling path/rights, en passant (incl. pinned-EP edge case), promotions.
+- **State integrity:** side-to-move flips; occupancy stays consistent.
+
+## Start-position reference counts
+
+  rnbqkbnr/pppppppp/8/8/8/8/PPPPPPPP/RNBQKBNR w KQkq - 0 1
+
+- d1 = **20**
+- d2 = **400**
+- d3 = **8,902**
+- d4 = **197,281**
+- d5 = **4,865,609**
+- d6 = **119,060,324**
+
+## Other Positions
+
+The chess programming wiki provides several other starting positions to test. They 
+can be found here.
+
+**PERFT Results & test positions:** <https://www.chessprogramming.org/Perft_Results>
+
 ## Future Work
 - Incorporate algorithms and evaluation methods for chess positions to enable an **AI player**.