Fix missing Apache-2.0 license file in the published crate

[musicinmybrain]

Add a symbolic link to the top-level LICENSE file.

What does this PR do?

The Apache 2.0 license says in section 4(a): “You must give any other recipients of the Work or Derivative Works a copy of this License”

However, prior to this PR, the published crate does not contain the license text:

$ tar -tzvf safetensors-0.4.1.crate
-rw-r--r-- 0/0             105 1969-12-31 19:00 safetensors-0.4.1/.cargo_vcs_info.json
-rw-r--r-- 0/0            1528 1969-12-31 19:00 safetensors-0.4.1/Cargo.toml
-rw-r--r-- 0/0            1040 2006-07-23 21:21 safetensors-0.4.1/Cargo.toml.orig
-rw-r--r-- 0/0           10566 2006-07-23 21:21 safetensors-0.4.1/README.md
-rw-r--r-- 0/0            1701 2006-07-23 21:21 safetensors-0.4.1/benches/benchmark.rs
-rw-r--r-- 0/0             184 2006-07-23 21:21 safetensors-0.4.1/src/lib.rs
-rw-r--r-- 0/0           16641 2006-07-23 21:21 safetensors-0.4.1/src/slice.rs
-rw-r--r-- 0/0           41672 2006-07-23 21:21 safetensors-0.4.1/src/tensor.rs

This PR adds a symbolic link in the safetensors/ directory to the top-level LICENSE file. This is derefenced by cargo publish, and after the PR, a copy of the top-level LICENSE is include in the crate as a normal file, satisfying the distribution requirements of the license.

$ cargo publish --dry-run
[…]
$ tar -tzvf ./target/package/safetensors-0.4.2-dev.0.crate
-rw-r--r-- 0/0             105 1969-12-31 19:00 safetensors-0.4.2-dev.0/.cargo_vcs_info.json
-rw-r--r-- 0/0            1534 1969-12-31 19:00 safetensors-0.4.2-dev.0/Cargo.toml
-rw-r--r-- 0/0            1046 2006-07-23 21:21 safetensors-0.4.2-dev.0/Cargo.toml.orig
-rw-r--r-- 0/0           11357 2006-07-23 21:21 safetensors-0.4.2-dev.0/LICENSE
-rw-r--r-- 0/0           10566 2006-07-23 21:21 safetensors-0.4.2-dev.0/README.md
-rw-r--r-- 0/0            1701 2006-07-23 21:21 safetensors-0.4.2-dev.0/benches/benchmark.rs
-rw-r--r-- 0/0             184 2006-07-23 21:21 safetensors-0.4.2-dev.0/src/lib.rs
-rw-r--r-- 0/0           16641 2006-07-23 21:21 safetensors-0.4.2-dev.0/src/slice.rs
-rw-r--r-- 0/0           41672 2006-07-23 21:21 safetensors-0.4.2-dev.0/src/tensor.rs

[Narsil]

Thank you for this