add basic readme

README.md

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+# vmt-parser
+
+Parser for source engine material files.
+
+## Usage
+
+```rust
+use vmt_parser::from_str;
+use std::fs::read_to_string;
+
+fn main() -> Result<(), Box<dyn std::error::Error>> {
+    let raw = read_to_string("material.vmt")?;
+    let material = from_str(&raw)?;
+    println!("texture: {}", material.base_texture());
+    Ok(())
+}
+```
+
+## Material support
+
+Because this crate focuses on extracting common rendering parameters from the
+materials, it only supports a fixed set of materials. If you need to parse a
+material that isn't supported by this crate, you can do a more manual parsing by
+using [vdf-reader](https://crates.io/crates/vdf-reader) directly.