Clarify specifications about custom regulations

[acc-is-sponge]

closes #4

Custom regulations (regulations that are not provided by MOD) are highlighted:

We use $(BeatSaberDir)/UserData/BeatSpeedrun/{CustomRegulations,CustomMapSets} directories for custom {regulations,mapsets}.

• This MOD automatically collects custom regulations from $(BeatSaberDir)/UserData/BeatSpeedrun/CustomRegulations
• custom: prefix is required for rules.mapSet setting in the regulation file: https://github.com/acc-is-sponge/beat-speedrun-regulations/commit/f897cc8582cdefdb025bae3df265a85866be8820

Don't cache MOD-provided regulations/mapsets anymore

$(BeatSaberDir)/UserData/BeatSpeedrun/{MapSets,Regulations} are no longer used.