Added Crankier docs (#9044)

Docs describing usage of Crankier

src/SignalR/perf/benchmarkapps/Crankier/Readme.md

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+# Crankier
+
+Load testing for ASP.NET Core SignalR
+
+## Commands
+
+### local
+
+The `local` command launches a set of local worker clients to establish connections to your SignalR server.
+
+```
+> dotnet run -- help local
+
+Usage:  local [options]
+
+Options:
+  --target-url <TARGET_URL>                   The URL to run the test against.
+  --workers <WORKER_COUNT>                    The number of workers to use.
+  --connections <CONNECTION_COUNT>            The number of connections per worker to use.
+  --send-duration <SEND_DURATION_IN_SECONDS>  The send duration to use.
+  --transport <TRANSPORT>                     The transport to use (defaults to WebSockets).
+  --worker-debug                              Provide this switch to have the worker wait for the debugger.
+```
+
+Notes:
+
+* `TARGET_URL` needs to be the route to your hub exposed by `UseSignalr` in your application.
+* `CONNECTION_COUNT` defaults to 10,000.
+* `SEND_DURATION_IN_SECONDS` defaults to 300
+* `WORKER_COUNT` defaults to 1
+
+#### Examples
+
+Attempt to make 10,000 connections to the `echo` hub using WebSockets and 10 workers:
+
+```
+dotnet run -- local --target-url https://localhost:5001/echo --workers 10
+```
+
+Attempt to make 5,000 connections to the `echo` hub using Long Polling
+
+```
+dotnet run -- local --target-url https://localhost:5001/echo --connections 5000 --transport LongPolling
+```