Adding contribution guide and enable copyright checks as a pre-commit hook

[hatemhelal]

This patch adds a contribution guide covering setting up a development environment for IPU development. This guide was initially developed for other projects (see hydronet-gnn). While reviewing the steps it occurred to me that we don't have a copyright check (required for OSS) so I've implemented a basic pre-commit hook that applies the flake8-copyright check.

Adds the following test-only dependencies:

• pre-commit
• flake8
• flake8-copyright

[awf]

Fab, thanks. Various doc proposals.
On re-looking, I like to separate the temporary shell variable and the "persistent" variable.