Add doc.go

Author: antonmedv

doc.go

@@ -0,0 +1,14 @@
+/*
+Package expr is an engine that can evaluate expressions.
+
+	// Evaluate expression on data.
+	result, err := expr.Eval("expression", data)
+
+	// Or precompile expression to ast first.
+	node, err := expr.Parse("expression")
+
+	// And run later.
+	result, err := expr.Run(node, data)
+
+*/
+package expr