Add mechanisms with electron cross section data (Cantera/cantera#1887)

[speth]

• Add new data files for air plasma and plasma-assisted methane combustion, used in two new examples introduced in Two-Term Boltzmann Equation solver cantera#1887 (plasma-eedf.py and nanosecond-pulse-discharge.py, respectively).
• Add README that explains the process for handling the two PRs required to add a new example to this repository and corresponding examples in the main Cantera repository.

[ischoegl]

With Cantera/cantera#1887 now merged, this should be completed and merged as well.

[speth]

@ischoegl - Sorry, we got this a little out of order -- this PR should have been merged before Cantera/cantera#1887 (as described in the new README). The main repo will now require a new commit to update the submodule after merging this.

[ischoegl]

@speth ... Thanks - since this was WIP, I did not pay attention to the text prior to merging the PR on the main repo.

Would merge, but I appear to not have write access to this repo.

[speth]

@ischoegl - I think I've got the permissions issue fixed for future PRs.