Add "experimenal" feature and require it for experimenal features

[notmandatory]

Experimental features:

• compact_filters
• descriptor_policy (new)
• any others?

For example:

// This function is only included when "experimental" and "compact_filters" features are enabled
#[cfg(all(feature = "experimental", feature = "compact_filters")]
#[cfg_attr(docsrs, doc(cfg(feature = "compact_filters")))]
pub mod compact_filters;
#[cfg(all(feature = "experimental", feature = "compact_filters")]
pub use self::compact_filters::CompactFiltersBlockchain;

... 

[RCasatta]

as discussed in #219 we are not going to change features that are experimental neither adding a experimental feature, just adding to docs they are experimental