Update instructions for Clojure bindings

[AlexChalk]

Description

While setting up the repository on my local machine, I came across some instructions that were out of date:

• Building everything from source was recommended if you want to make changes to any of the code, but I understand from the mxnet Slack channel that using the nightly snapshots of the Scala package jar is adequate for making changes to the Clojure code.
• Updating the project.clj file was part of the instructions for building everything from source, but this is no longer necessary.
• Similarly the project.clj file lists [org.apache.mxnet/mxnet-full_2.11 "INTERNAL"] as used for CI builds, when it is also used for local builds of the Scala package jar.

This PR updates those instructions.

Checklist

Essentials

Please feel free to remove inapplicable items for your PR.

• Changes are complete (i.e. I finished coding on this PR)
• Code is well-documented:
• To the my best knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change

Changes

• Update the Clojure package's README.md.
• Update a comment in the Clojure package's project.clj.

[gigasquid]

Thanks for your contribution to make the documentation better @adc17 😸