Develop categories for known source gaps

[bambooforest]

See #332

[camoverride]

@bambooforest - will CLDF be the main resource for tracking known issues with the phoible source material? I'm wondering if I should invest time digging into this.

[bambooforest]

@camoverride -- it would be great to get some feedback. we do all of our dev work in this repository, so this is the most up-to-date data with corrections, etc.:

https://github.com/phoible/dev/tree/master/data

so you should use this.

when we've made significant changes that warrant a new version, we generate the new version in this repository and generate the CLDF data, which gets archived in Zenodo as CLDF and also here for the website:

https://github.com/cldf-datasets/phoible

[bambooforest]

Some things to keep in mind here (not exhaustive):

• UPSID does not have allophones (but does mark marginals)
• AusPhon consistently does include any marginals