Document write operations in README

[rubensworks]

Now that #6 has been merged, we should document the usage of the following methods in the README:

• add
• delete
• set
• replace

[scenaristeur]

Ldflex need updated doc !!!
Please https://forum.solidproject.org/t/remove-friend-403-user-unauthorized/3358

[RubenVerborgh]

@scenaristeur All help appreciated.
For instance, in the form of PRs, or https://github.com/sponsors/RubenVerborgh

[jeswr]

🎉 This issue has been resolved in version 2.15.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

Your semantic-release bot 📦🚀