Simplify client building and installation instructions.

[trishankatdatadog]

Hello there! :)

I'm just simplifying the building and installation instructions to what worked for me.

Let me know what you think!

[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "trishank_kuppusamy/simplify-client-installation-instructions" git@github.com:trishankatdatadog/notary.git somewhere
$ cd somewhere
$ git commit --amend -s --no-edit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[docker-jenkins]

Can one of the admins verify this patch?

[endophage]

Jenkins test this please

[endophage]

LGTM pending Jenkins (which should bump the codecov too)

[endophage]

@trishankatdatadog can you go back and add the signed off by line to the first of the two commits please.

[trishankatdatadog]

done :)

[endophage]

Jenkins test this please

[cyli]

LGTM :)

[trishankatdatadog]

Thanks @cyli! Do I need to do anything else here?

[cyli]

@trishankatdatadog Nope, this is great, thank you!