Add README for buffer pool visualizer

src/buffer_pool_visualizer/README.md

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+# Buffer Pool Visualizer
+
+A visual debugging tool for the Hermes `BufferPool` and `MetadataManager`.
+
+To run, start a Hermes instance and then run `make viz` from the build
+directory. It can also visualize a crashed Hermes run if the shared memory
+segment still exists.
+
+To enable in the build, set `HERMES_BUILD_BUFFER_POOL_VISUALIZER=ON` in CMake.
+In order to visualize the `MetadataManager`, Hermes must be built with the CMake
+option `HERMES_DEBUG_HEAP=ON`.
+
+## Dependencies
+
+[SDL](https://www.libsdl.org/) is the only additional dependency. On Ubuntu,
+`sudo apt install libsdl2-dev`.
+
+## Controls
+
+| Key | Functionality                                          |
+|-----|--------------------------------------------------------|
+| 0-9 | If viewing the `BufferPool`, display Tier N            |
+| 0   | If viewing the `MetadataManager`, display the ID heap  |
+| 1   | If viewing the `MetadataManager`, display the map heap |
+| b   | View the `BufferPool`                                  |
+| c   | Print a count of all buffers and slabs                 |
+| f   | Print the sizes of the buffer free lists               |
+| m   | View the `MetadataManager`                             |
+| r   | Reload the shared memory file                          |
+| s   | Save a bitmap of the current view                      |
+| ESC | Exit the visualizer                                    |
+