Link to function documents, make sure they render correctly

[peterdesmet]

• [x] In README, link to function documentation. Done in https://github.com/enram/vptstools/pull/50/commits/2cf980bc56df977cfac843d2e60ca47c4ae13765
• [x] In README, don't list config variables (should be in function documentation). Done in https://github.com/enram/vptstools/pull/50/commits/2cf980bc56df977cfac843d2e60ca47c4ae13765
• [x] Make sure function documentation for transfer_baltrad renders correctly (too few or many line breaks atm)
• [x] Make sure function documentation for vph5_to_vpts renders (currently just a title)

See https://github.com/enram/vptstools/pull/50 for a start