Added Requirement

[VeeeneX]

Just to avoid #190

[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

[ascandella]

Let's make this a bit more descriptive:

* [Arcanist](https://github.com/phacility/arcanist) is installed on the Jenkins nodes where the tests will be run (e.g. not just the master) an the `arc` binary is in `$PATH` or configured explicitly in the global settings.

or something

[VeeeneX]

Sure!

[coveralls]

Coverage remained the same at 91.323% when pulling 39e2546 on VeeeneX:patch-1 into 69516ab on uber:master.

[ascandella]

typo: and the ...

Also we should note that this is only required if you use the "Apply Phabricator Differential" part of the plugin. Some people just use the post-build action (e.g. to report data to harbormaster, which can be used either with staging areas or if you're running builds on diffusion commits). But we can clarify that later. For now just fix the typo then

Thanks!

[coveralls]

Coverage remained the same at 91.323% when pulling 39e2546 on VeeeneX:patch-1 into 69516ab on uber:master.

[ascandella]

Thanks!