add full feature list to `./README.md`

[alejandro-vaz]

add all the features in Cargo.toml with a simple explanation to the README (one paragraph each, don't go crazy)

this allows us in the future to remove the massive doc-comment from lib.rs

there is already a simple explanation in the first 50 lines of lib.rs, but not all features there might exist anymore, and there might be new ones as well

current features are:

• std
• specialization
• may_dangle
• serde
• internals
• bytes
• malloc_size_of