testdata: add description to signers.ts

Author: ScottyPoi

packages/testdata/src/signers.ts

@@ -1,5 +1,11 @@
 import { Address, hexToBytes } from '@ethereumjs/util'
 
+/**
+ * Commen set of Signers for internal test construction.
+ * Sets of privateKeys, publicKeys, and addresses
+ * Use these signers in test construction instead of hardcoding or constructing new signers.
+ */
+
 export type Signer = {
   address: Address
   privateKey: Uint8Array