Add README & CONTRIBUTION documenation

[bchalios]

We should add a README file on how to use vAccel and a contribution guide with all the processes that need to be followed in order to contribute to vAccel, both for internal and external use.

Component(s) that need(s) improvement

1. vaccel
2. Maybe at this point we should add the equivalent files to sub-modules (?)

Describe the improvement

We mainly need a README file that describes how vAccel can be used end to end. This would include building vAccel or downloading a particular version of it and setting up the environment for running an end-to-end example.

We also need a contributor's guide to use as a reference for our internal development and (hopefully) external contributions.

I will update this issue as I have a more concrete idea of the structure of the documents with TODOs