Add 'Getting Started' section to README with dependency and build instructions

[GrahamBenHarper]

Building ROL yesterday was incredibly easy, but I think it would also be nice if there was a small section in the README which echoes @aj463's instructions. This adds a brief "Getting Started" section to the README with the dependencies and cmake command written out for posterity.

Maybe @dridzal would also want to look at this?

[aj463]

This section is a nice idea and could even be added to the ROL website. Adding @dridzal as a reviewer for his feedback. The link below shows what the updated GitHub page would look like:

https://github.com/GrahamBenHarper/rol/tree/readme-getting-started

[dridzal]

This looks good, for now. Note that there are some instructions in the context of building with Trilinos, here:

https://rol.sandia.gov/user-resources/

We should probably update the website to include both standalone and Trilinos instructions. Similarly, we can update the README file to discuss building with Trilinos, as that turns out to be trivial, via removing the rol directory and soft-linking the standalone repo's rol directory (I hear that there may be a way to do this without soft-linking, per Kokkos' workflow).