Squashfs documentation seems to give misleading information about supported compressions

[nanonyme]

Does this project have any filter for compressions that would justify the documentation data? I couldn't find such a filter. Documentation however says supported compressions are gzip (default), lzo, xz or none but

1. squashfs-tools by default only gzip is enabled
2. it is in addition to listed compressions also possible to build squashfs-tools with lz4 and zstd support

If there is no such filter, minimal fix might be to mention support of exact compressions depends on what provided squashfs-tools supports and it could be more or fewer than listed.

[michaelolbrich]

I've created #272 to clarify the documentation.