Explain the compatibility filters for react native

[dmfrancisco]

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The new "Web" and "Expo" react-native filters come from the native.directory project. They have been requested in the past (#532) and may help people that are looking for packages that are compatible with expo for example.

The problem now is that they are not explained and the counters make it look like the majority of packages are not compatible with Expo, which is not the case — they simply haven't been added to the native.directory catalog.

Consider adding a help text above these filters.