Build instructions reference pulling in polkadot branch rococo-v1

[jessdecker]

This currently doesn't build, and a related issue mentions this isn't supported. paritytech/subport#229

[bernardoaraujor]

Another point where README seems outdated is that it only instructs me to run cargo build --release for the polkadot repo.
As a consequence, I kept running into this error:

Parachain binary does not exist:  /home/bear/develop/parity/benchmark-report/bin/adder-collator

It took me a while to realize how to compile adder-collator from the polkadot repo:

$ cargo build --release -p test-parachain-adder-collator

Ideally, README should instruct the user how to build adder-collator after they build the polkadot binary.
That is, depending on how #131 moves forward.