doc(license): fix format of license file (#87)

This commit follows 0dbf75c

Making possible to be detected by tools.
For example, currently GitHub does not detect this
repository as an Apache-2.0 project.

New LICENSE file has been obtained with

```bash
curl -L https://www.apache.org/licenses/LICENSE-2.0.txt -o LICENSE
```

Added the NOTICE file as written in the following links

https://www.apache.org/legal/apply-license.html
https://infra.apache.org/licensing-howto.html

Finally checked the result with

https://github.com/licensee/licensee/tree/v9.18.0

Before
---

```plaintext
License:        NOASSERTION
Matched files:  LICENSE, README.md, Cargo.toml
LICENSE:
  Content hash:  6836f6084c5e24bf671db6bf35b3442c5f604b93
  License:       NOASSERTION
  Closest non-matching licenses:
    ECL-2.0 similarity:     82.54%
    Apache-2.0 similarity:  70.61%
    OSL-3.0 similarity:     51.56%
README.md:
  Content hash:  1a017b26702ef2ed885dcdacfc41f29c8893ee57
  Confidence:    90.00%
  Matcher:       Licensee::Matchers::Reference
  License:       Apache-2.0
  Closest non-matching licenses:
    0BSD similarity:   8.51%
    ISC similarity:    6.67%
    WTFPL similarity:  6.11%
Cargo.toml:
  Confidence:  90.00%
  Matcher:     Licensee::Matchers::Cargo
  License:     Apache-2.0
```

After
---

```plaintext
License:        Apache-2.0
Matched files:  LICENSE, README.md, Cargo.toml
LICENSE:
  Content hash:  bec905d850e7f5dc2e2db78a950d4a9db560a0b8
  Confidence:    100.00%
  Matcher:       Licensee::Matchers::Exact
  License:       Apache-2.0
README.md:
  Content hash:  1a017b26702ef2ed885dcdacfc41f29c8893ee57
  Confidence:    90.00%
  Matcher:       Licensee::Matchers::Reference
  License:       Apache-2.0
  Closest non-matching licenses:
    0BSD similarity:   8.51%
    ISC similarity:    6.67%
    WTFPL similarity:  6.11%
Cargo.toml:
  Confidence:  90.00%
  Matcher:     Licensee::Matchers::Cargo
  License:     Apache-2.0
```