Improvements for README.md

[impiyush83]

Readme file for local installations -> mongodb server setup is only for ubuntu not for mac

For setup of mongodb for mac the commands are :

brew install mongo
sudo mkdir -p /data/db
whoami
sudo chown <output_of_whoami> /data/db

Open ./bashrc or .zshrc file and copy paste the contents below

export MONGO_PATH=<path_to_mongodb>
export PATH=$PATH:$MONGO_PATH/bin

To run mongod instance :
mongod
Kill all instances of mongod in mac :
mongo admin --eval "db.shutdownServer()”

[impiyush83]

@mariobehling I have raised the PR for the above changes. This is my first opensource discussion so please guide me.