Better support approaches for resolving validation issues, in general

[carlhiggs]

Following #415 (in particular this post), @MelanieLowe and I proposed a series of responses that we could recommend when issues are identified with representation of urban features when conducting validation following analysis using our software:

1. Exceptions: Allow exceptions for specific regions for criteria within their configuration file
2. Contributions: Encourage contributions to OpenStreetMap where more comprehensive tagging would be appropriate
3. Customisation: Allow configuration of custom data
4. Methodological: Modify the default ways of identifying features of interest, following sensitivity analyses to understand implications for a range of urban contexts to be sure that this provides a net improvement
5. Acknowledgement: Acknowledge specific issues as limitations when reporting data

Approaches 1 and 3 can be better supported, and once this typology of approaches is agreed upon, we should add it into the FAQ (https://healthysustainablecities.github.io/software/#Frequently-Asked-Questions).