add discussion about workers-executors
update pdfs
fix typo by zzl0
update pdfs
update readme
Update 7-Broadcast.md

fix typo
Update 7-Broadcast.md
Update 6-CacheAndCheckpoint.md

fix typo
Merge pull request #6 from 397090770/patch-3

they an => they can
Merge pull request #4 from 397090770/patch-1

fix typo
Merge pull request #5 from 397090770/patch-2

fix typo
update readme
fix typo
fix typo
fix typo
Update 5-Architecture.md
Merge pull request #7 from ericsahit/patch-1

fix typo 'driver'
modify the graph of groupByKey
Merge branch 'master' of https://github.com/JerryLead/SparkInternals

modify the graph of 'groupByKey'
modify the figure of cogroup()