Added a contributors section to README.md
Added a contributors section to README.md

In preparation for accepting GitHub pull requests this new README
section outlines the general criteria that the leveldb project owners
will use when accepting external (and internal) project contributions.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=111349899

matehat/leveldb,yummyliu/leveldb,yuwolf/leveldb,norton/leveldb,jacklicn/leveldb,fuchsia-mirror/third_party-leveldb,dwing4g/leveldb,bitcoin-core/leveldb,WeiHaocheng/LevelDBBuffer,google/leveldb,jacklicn/leveldb,xuzhezhaozhao/leveldb,Mojang/leveldb-mcpe,yuwolf/leveldb,zxylvlp/leveldb,Mojang/leveldb-mcpe,xuzhezhaozhao/leveldb,bitcoin/leveldb,alexxvk/leveldb,cmumford/leveldb,dwing4g/leveldb,cmumford/leveldb,PFZheng/leveldb,luke-jr/leveldb,google/leveldb,olzhabay/PMIndexDB,PFZheng/leveldb,alexxvk/leveldb,olzhabay/PMIndexDB,likema/leveldb,tianhuhui/leveldb,bitcoin-core/leveldb,tianhuhui/leveldb,zxylvlp/leveldb,WeiHaocheng/LevelDBBuffer,matehat/leveldb,luke-jr/leveldb,xuerenlv/leveldb,norton/leveldb,tianhuhui/leveldb,yummyliu/leveldb,alexxvk/leveldb,Jianbo-Yang/leveldb,bitcoin/leveldb,jacklicn/leveldb,likema/leveldb,dwing4g/leveldb,fastogt/leveldb,bitcoin/leveldb,betterdenger/leveldb,PFZheng/leveldb,olzhabay/PMIndexDB,WeiHaocheng/LevelDBBuffer,Mojang/leveldb-mcpe,matehat/leveldb,bitcoin-core/leveldb,fuchsia-mirror/third_party-leveldb,norton/leveldb,Jianbo-Yang/leveldb,fastogt/leveldb,xuerenlv/leveldb,xuerenlv/leveldb,yuwolf/leveldb,zxylvlp/leveldb,betterdenger/leveldb,Jianbo-Yang/leveldb,yummyliu/leveldb,betterdenger/leveldb,xuzhezhaozhao/leveldb