Add a link to Apache BookKeeper which also uses gPRC

[sijie]

Apache BookKeeper uses gRPC in its table service, thus adding the link to it.

Signed-off-by: Sijie Guo sijie@apache.org

[thelinuxfoundation]

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[sijie]

I just signed the CLA :)

[carl-mastrangelo]

@sijie Can you sync to head and resolve the merge conflict?

[sijie]

thank you @carl-mastrangelo

I just rebased to master HEAD

[carl-mastrangelo]

@sijie Merged, Thanks!