Update Guava README to mention 21.0-rc1.
Update Guava README to mention 21.0-rc1.

Also update headers to ATX style.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=143120092

google/guava,rgoldberg/guava,EdwardLee03/guava,EdwardLee03/guava,xasx/guava,google/guava,mosoft521/guava,typetools/guava,google/guava,berndhopp/guava,xasx/guava,rgoldberg/guava,mosoft521/guava,DavesMan/guava,DavesMan/guava,Xaerxess/guava,typetools/guava,mosoft521/guava,Xaerxess/guava,berndhopp/guava