Remove buck2 references from examples/portable/README.md (#4027)

Summary:
Change the example to use cmake instead, and show some example output.

Pull Request resolved: #4027

Test Plan: Ran the new instructions in the README.md after running `./install_requirements.sh` to install the `executorch` python module.

Reviewed By: Gasoonjia

Differential Revision: D58850983

Pulled By: dbort

fbshipit-source-id: d73ea0b570713d6fc89706c591f5c250eb529a87

Author: dbort

examples/portable/README.md

@@ -44,9 +44,37 @@ Use `-h` (or `--help`) to see all the supported models.
 3. Once we have the model binary (`.pte`) file, then let's run it with ExecuTorch runtime using the `executor_runner`.
 
 ```bash
-buck2 run examples/portable/executor_runner:executor_runner -- --model_path ./mv2.pte
+# Build the tool from the top-level `executorch` directory.
+(rm -rf cmake-out \
+    && mkdir cmake-out \
+    && cd cmake-out \
+    && cmake -DEXECUTORCH_PAL_DEFAULT=posix ..) \
+  && cmake --build cmake-out -j32 --target executor_runner
+
+# Run the tool on the generated model.
+./cmake-out/executor_runner --model_path mv2.pte
 ```
 
+This will run the model with all input tensor elements set to `1`, and print
+the outputs. For example:
+```
+I 00:00:00.004885 executorch:executor_runner.cpp:73] Model file mv2.pte is loaded.
+I 00:00:00.004902 executorch:executor_runner.cpp:82] Using method forward
+I 00:00:00.004906 executorch:executor_runner.cpp:129] Setting up planned buffer 0, size 18652672.
+I 00:00:00.007243 executorch:executor_runner.cpp:152] Method loaded.
+I 00:00:00.007490 executorch:executor_runner.cpp:162] Inputs prepared.
+I 00:00:06.887939 executorch:executor_runner.cpp:171] Model executed successfully.
+I 00:00:06.887975 executorch:executor_runner.cpp:175] 1 outputs:
+Output 0: tensor(sizes=[1, 1000], [
+  -0.50986, 0.300638, 0.0953877, 0.147722, 0.231202, 0.338555, 0.20689, -0.057578, -0.389269, -0.060687,
+  -0.0213992, -0.121035, -0.288955, 0.134054, -0.171976, -0.0603627, 0.0203591, -0.0585333, 0.337855, -0.0718644,
+  0.490758, 0.524144, 0.197857, 0.122066, -0.35913, 0.109461, 0.347747, 0.478515, 0.226558, 0.0363523,
+  ...,
+  -0.227163, 0.567008, 0.202894, 0.71008, 0.421649, -0.00655106, 0.0114818, 0.398908, 0.0349851, -0.163213,
+  0.187843, -0.154387, -0.22716, 0.150879, 0.265103, 0.087489, -0.188225, 0.0213046, -0.0293779, -0.27963,
+  0.421221, 0.10045, -0.506771, -0.115818, -0.693015, -0.183256, 0.154783, -0.410679, 0.0119293, 0.449714,
+])
+```
 
 ## Custom Operator Registration