Add docs for performance considerations

Author: onethumb

src/structs.rs

@@ -42,6 +42,9 @@ impl CrcCalculator for Calculator {
 impl CrcParams {
     /// Creates custom CRC parameters for a given set of Rocksoft CRC parameters.
     ///
+    /// Generates the folding keys on-demand, which has a slight performance penalty. For
+    /// performance-critical applications, consider generating the keys once and reusing them.
+    ///
     /// Does not support mis-matched refin/refout parameters, so both must be true or both false.
     ///
     /// Rocksoft parameters for lots of variants: https://reveng.sourceforge.io/crc-catalogue/all.htm