Update README to mention Bazelisk (#5732)
Update README to mention Bazelisk (#5732)

## What is the goal of this PR?

Ensure whoever wants to build Grakn Core from sources uses the correct Bazel version.

## What are the changes implemented in this PR?

Mention Bazelisk and `.bazelversion` in `README`
graknlabs/grakn,lolski/grakn,lolski/grakn,graknlabs/grakn,lolski/grakn,graknlabs/grakn,graknlabs/grakn,lolski/grakn