hash: Add readme

[joii2020]

Based on community feedback, attempts to use ckb-hash in contracts have failed to compile due to missing usage instructions.
This update adds a brief guide, and developers can find more details on crates.io and docs.rs.

[zhangsoledad]

failures:

---- util/hash/src/lib.rs - (line 9) stdout ----
error: expected one of `.`, `;`, `?`, `}`, or an operator, found `,`
 --> util/hash/src/lib.rs:10:25
  |
3 | default-features = false, features = ["ckb-contract"]
  |                         ^ expected one of `.`, `;`, `?`, `}`, or an operator

error: aborting due to 1 previous error

Couldn't compile the test.

failures:
    util/hash/src/lib.rs - (line 9)

@joii2020

The newly added documentation comments were run as documentation tests(https://doc.rust-lang.org/rustdoc/write-documentation/documentation-tests.html) and failed, so they need to be modified a bit.