Update master readme projection section

[TomBearpark]

The master readme currently states (under 'Step 3 - Project Future Impacts of Climate Change') that "Code for this step is not currently in this repo.".

Code for this step is currently in the projection branch (see PR here).

However, we are not planning on merging this branch until we have finalized our preferred projection model, made James' codes and data public, such that the code for running projections is usuable for a non-CIL member (i.e. doesn't have dependencies on sac/BRC).

When this is done - we should:

• Merge the projection PR

• Update the master readme to reflect the fact that users can run projections.

edit: tagging @aarode