Cleans up README and simplifies build steps

1. Remove leftover mentions of `make` in the context of building ORCA
1. Because ninja is parallel by default, remove mentions of how to
   parallelize the build

* Simplify build steps in README

Noticeably, we no longer require the two most hated steps:
cd-after-mkdir-build. Instead `cmake` will directly mkdir the build
directly if it doesn't exist.

* [ci skip]
This fixes #248

Signed-off-by: NEkta Khanna <ekhanna@pivotal.io>

Now that we all are using it.
[ci skip]

[ci skip]
Signed-off-by: NJemish Patel <jpatel@pivotal.io>

Signed-off-by: NDhanashree Kashid <dkashid@pivotal.io>

Signed-off-by: NHaiSheng Yuan <hyuan@pivotal.io>

Remove the % for easier copy to terminal.