Keep Rust README minimal

[ilammy]

We have three to four places with assorted documentation on Themis and this is getting a bit hard to maintain consistency between them. Let's keep Rust README minimal so that it does not require frequent changes. Just refer to relevant pieces of documentation from it and avoid any detailed explanations.

[karenswry]

Please substitute for the following.
Or grant me more permissions on the branch the next time 👿 I'll edit it without bothering you.

Quickstart

Rust-Themis is a wrapper so it requires the native Themis library to be installed for it to work. Please refer to the quickstart guide for installation instructions.

You can read the docs:

• General Wiki-documentation for Themis
• In-depth documentation on Cossack Labs Documentation server
• Rust API documentation on docs.rs
• Changelog for versions on GitHub

Start with experimenting with the examples or take a look at the tests to get a feel for how Themis can be used.

Licensing

The code is distributed under Apache 2.0 license.

[vixentael]

@karenswry no worries, all these docs are scheduled to be reviewed by you during release preparations.