Add LCSH to the list of subject schemes

[We currently document](https://github.com/readium/webpub-manifest/tree/master/contexts/default#subjects) URIs to identify the following subject schemes:

- BIC
- BISAC
- CLIL
- THEMA

In the library world, there are other subject schemes that are widely used, which is why I believe that LCSH should be added.