Add basic testing guide to README.

[simi]

• extracted from Binary COPY is never used for tables with array columns (PostgresType.oid is never populated) #431 (comment)

[staticlibs]

@simi

Thanks for the PR! This can be merged without the CI run, just please replace the linux_amd64 references there with the platform placeholder.

[simi]

Updated. What about the DUCKDB_PLATFORM approach?

[staticlibs]

Thanks!