Update needed in Readme and update-config.sh

[dsv9]

I ran into permission and 'No such filre or directory' errors when installing and I have a few proposals

In the readme.md, the update-config.sh needs permission to be executed as well, so:
chmod +x start-mainnet-node.sh start-testnet-node.sh should inlcude update-config.sh as well, altough the culprit here might've been the next thing.

In the update-config.sh file, the first set of slashes from each wget needs to be removed, because update-config.sh would be run from the folder itself.
so: wget -O /mainnet/mainnet-config.json https://hydra.iohk.io/job ...
becomes: wget -O mainnet/mainnet-config.json https://hydra.iohk.io/job

I was getting No such file or directory errors and tried converting them with dos2unix, installed i386 architectures and whatnot. Then I realised the forward slahsed at the beginning of the directory.