core: add ABOUT for quick verification and notes

ABOUT.md

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+
+# Backend internal interview (rust) - improved  
+
+## Verification
+
+From [input.txt](input.txt) compare output with [output_expected.txt](output_expected.txt)
+
+```bash
+cargo run -q -- -i input.txt -q
+```
+
+From tests
+
+```rust
+cargo test test_expected_metrics
+```
+
+**test_expectd_metrics** can be found at the end of [tests.rs](src/tests.rs)
+
+> DEFAULT_INPUT_PATH and DEFAULT_REG_EXP values can be found in [defs.rs](src/defs.rs)
+
+### For parallel processing
+
+A second file [input_2.txt](input_2.txt) has been created and included in [config.toml](config.toml)
+
+```rust
+cargo run -q
+```
+
+It the same as:
+
+```rust
+cargo run -q -- -c config.toml
+```
+
+> [!NOTE]
+> [config.toml](config.toml) is DEFAULT_CONFIG_PATH if does not exists it will only use DEFAULT_INPUT_PATH [input.txt](input.txt)
+
+## Read Documentation
+
+Build documentation and browse to **main** page
+
+```rust
+cargo doc --no-deps --open
+```
+
+Source code contains doc text.  
+
+## How to use
+
+For more explanations use [how to use](howto.md)