Guide for adding new compressors

[treigerm]

We need a guide that tells people how to add their own compressor to the benchmark. @juntyr pointed out that there are actually multiple ways to add a new compressor:

• Simply hack together a Python script that produces the necessary decompressed.zarr files
• Implement a new numcodecs-wasm codec
• Implement a new numcodecs codec ? Does this work with the current interface? I guess some of the performance measurements might not work anymore?