Created Contributing checkers (markdown)

Contributing-checkers.md

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+```py
+from typing import Dict, Optional
+
+from loguru import logger
+from pywhat import identifier
+
+from ciphey.iface import Checker, Config, ParamSpec, T, registry
+
+
+@registry.register
+class GTestChecker(Checker[str]):
+
+    """
+    G-test of fitness, similar to Chi squared.
+    """
+
+    def check(self, text: T) -> Optional[str]:
+        logger.trace("Trying PyWhat checker")
+        returned_regexes = self.id.identify(text)
+        if returned_regexes["Regexes"] > 0:
+            return returned_regexes["Regexes"][0]["Regex Pattern"]["Name"]
+        return None
+
+    def getExpectedRuntime(self, text: T) -> float:
+        # TODO: actually bench this
+        return 4e-7 * len(text)
+
+    def __init__(self, config: Config):
+        super().__init__(config)
+        self.id = identifier.Identifier()
+
+    @staticmethod
+    def getParams() -> Optional[Dict[str, ParamSpec]]:
+        pass
+
+```
+
+Checkers return a string (with what it found) or None if it fails.