docs: Update README.md and godoc for Boolean-to-Number coercion

- Added documentation section to README.md explaining automatic coercion
- Included usage examples: '5 3 == 1 +', '0 false +', 'true 2 *', '9 3 > 4 5 < +'
- Updated Value type godoc to explain coercion behavior (true→1, false→0)
- toNumber() function already had good documentation

The coercion allows boolean results to be used directly in arithmetic operations,
enabling expressions like '5 3 == 1 +' where '5 3 ==' produces false=0, then '0 + 1' produces 1.

1 files changed, 8 insertions, 0 deletions

diff --git a/internal/rpn/variables.go b/internal/rpn/variables.go
index de98e2f..5214403 100644
--- a/internal/rpn/variables.go
+++ b/internal/rpn/variables.go
@@ -14,6 +14,14 @@ var (
 )
 
 // Value represents a variant type that can hold either a number (float64) or a boolean.
+//
+// When used in arithmetic operations, boolean values are automatically coerced:
+//   - true → 1
+//   - false → 0
+//
+// This allows boolean results from comparison operations to be used directly in
+// arithmetic expressions (e.g., "5 3 == 1 +" where "5 3 ==" produces false=0,
+// and "0 + 1" produces 1).
 type Value struct {
 	isBool bool
 	boolVal bool