PARQUET-270: Adds a legend for meta output to readme.md
PARQUET-270: Adds a legend for meta output to readme.md

Author: Brett Stime <brett.stime@nativex.com>

Closes #178 from w3iBStime/patch-2 and squashes the following commits:

c6c6898 [Brett Stime] Makes meta legend more descriptive
6d32bc3 [Brett Stime] Update README.md
b1e38aa [Brett Stime] Adds a legend for meta output to readme.md

SinghAsDev/parquet-mr,sircodesalotOfTheRound/parquet-mr,cchang738/parquet-mr,nguyenvanthan/parquet-mr,sircodesalotOfTheRound/parquet-mr,zhenxiao/parquet-mr,winningsix/incubator-parquet-mr,nguyenvanthan/parquet-mr,spena/parquet-mr,nguyenvanthan/parquet-mr,apache/parquet-mr,coughman/incubator-parquet-mr,Zariel/parquet-mr,laurentgo/parquet-mr,jaltekruse/parquet-mr-1,laurentgo/parquet-mr,jaltekruse/parquet-mr-1,cchang738/parquet-mr,nitin2goyal/parquet-mr-1,Zariel/parquet-mr,coughman/incubator-parquet-mr,apache/parquet-mr,SinghAsDev/parquet-mr,rdblue/parquet-mr,piyushnarang/parquet-mr,nezihyigitbasi-nflx/parquet-mr,MickDavies/incubator-parquet-mr,piyushnarang/parquet-mr,SaintBacchus/parquet-mr,apache/parquet-mr,sworisbreathing/parquet-mr,SaintBacchus/parquet-mr,HyukjinKwon/parquet-mr,saucam/incubator-parquet-mr,rdblue/parquet-mr,winningsix/incubator-parquet-mr,MickDavies/incubator-parquet-mr,DataDog/parquet-mr,sworisbreathing/parquet-mr,HyukjinKwon/parquet-mr-1,HyukjinKwon/parquet-mr-1,nezihyigitbasi-nflx/parquet-mr,DataDog/parquet-mr,Zariel/parquet-mr,sworisbreathing/parquet-mr,DataDog/parquet-mr,HyukjinKwon/parquet-mr,nezihyigitbasi-nflx/parquet-mr,HyukjinKwon/parquet-mr-1,SaintBacchus/parquet-mr,jaltekruse/parquet-mr-1,cchang738/parquet-mr,MickDavies/incubator-parquet-mr,saucam/incubator-parquet-mr,zhenxiao/parquet-mr,SinghAsDev/parquet-mr,nitin2goyal/parquet-mr-1,saucam/incubator-parquet-mr,rdblue/parquet-mr,zhenxiao/parquet-mr,sircodesalotOfTheRound/parquet-mr,HyukjinKwon/parquet-mr,winningsix/incubator-parquet-mr,spena/parquet-mr,piyushnarang/parquet-mr,nitin2goyal/parquet-mr-1,spena/parquet-mr,coughman/incubator-parquet-mr,laurentgo/parquet-mr