Remove object_store references in Readme.md

[alamb]

Which issue does this PR close?

• Part of [EPIC] Port object_store content from arrow-rs repository arrow-rs-object-store#2
• Related to Remove object_store from arrow-rs repository #7316
• Related to [DISCUSSION] Proposal move object_store to its own github repo? #6183

Rationale for this change

The current Readme.md contains references to object_store which is being moved to its own repo, so let's update the main readme as well

What changes are included in this PR?

Remove object_store related content. This is added to the object store repo in this PR:

• Update README.md and Contributing guidelines arrow-rs-object-store#8

Are there any user-facing changes?

README is updated

[tustvold]

I think we should link to the new home of object_store as a related crate, it is as related as the likes of datafusion or ballista IMO, if not more so given it has the same governing PMC

[alamb]

I think we should link to the new home of object_store as a related crate, it is as related as the likes of datafusion or ballista IMO, if not more so given it has the same governing PMC

Thank you -- this is a good idea -- done in dfcaa64

[alamb]

Thanks again @tustvold