Add README for the InMemoryTransport sample

samples/InMemoryTransport/README.md

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+# In-Memory Transport Sample
+
+Demonstrates connecting an MCP client and server in the same process using stream-based
+transports over in-memory pipes (`System.IO.Pipelines`) — no child process, no network.
+
+The server is created directly with `McpServer.Create`, without a host or dependency
+injection container, and exposes a single `Echo` tool defined from a delegate with
+`McpServerTool.Create`. The client connects over the same pipe pair with
+`StreamClientTransport`, lists the server's tools, and invokes the tool.
+
+This pattern is useful for testing MCP servers, embedding a server inside a larger
+application, or running a client and server in the same process without transport overhead.
+See [Transports: In-memory transport](../../docs/concepts/transports/transports.md) for more
+background.
+
+## Run
+
+```bash
+dotnet run --project samples/InMemoryTransport/InMemoryTransport.csproj
+```
+
+Expected output:
+
+```
+Tool Name: Echo
+
+Echo: Hello World
+```
+
+## Key files
+
+- [`Program.cs`](Program.cs) — the entire sample: pipe setup, server creation, client
+  connection, tool listing, and tool invocation.
+
+## Notes
+
+- `StreamServerTransport` and `StreamClientTransport` work with any `Stream`. This sample
+  wires them to a pair of `Pipe` instances, one per direction; the client's output stream is
+  the server's input stream and vice versa.
+- The server is started with a fire-and-forget `server.RunAsync()` because both endpoints
+  live in the same process. `await using` on the server and client ensures both are disposed
+  when the program exits.