docs: we should provide a Compatibility matrix on importer api version, importer version, NebulaGraph version

[wey-gu]

as titled, on docs and readme.

• importer api v2( importer version 2.x & 3.x)
• importer api v3( importer version 4.x)

[wey-gu]

see:

• Can not import data into NebulaGraph nebula-contrib/NebulaGraph-Bench#85