Adds README

README.md

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+# ERDP
+
+ERDP is a very small Rust crate to help with error displaying. If you use `std::fmt::Display` to display a `std::error::Error` like the following code:
+
+```rust
+use std::fs::File;
+use std::path::{Path, PathBuf};
+use thiserror::Error;
+
+fn main() {
+    let path = PathBuf::from("config.json");
+
+    if let Err(e) = load_config(&path) {
+        eprintln!("Failed to load {}: {}.", path.display(), e);
+    }
+}
+
+fn load_config(path: &Path) -> Result<(), MyError> {
+    let file = match File::open(path) {
+        Ok(v) => v,
+        Err(e) => return Err(MyError::OpenFileFailed(e)),
+    };
+
+    Ok(())
+}
+
+#[derive(Debug, Error)]
+enum MyError {
+    #[error("couldn't open the specified file")]
+    OpenFileFailed(#[source] std::io::Error),
+}
+```
+
+What you get is just a message from a top-level error:
+
+```
+Failed to load config.json: couldn't open the specified file.
+```
+
+With this crate you can use `display` method on the error value like:
+
+```rust
+use erdp::ErrorDisplay;
+
+eprintln!("Failed to load {}: {}.", path.display(), e.display());
+```
+
+Then the output will change to something like:
+
+```
+Failed to load config.json: couldn't open the specified file -> No such file or directory.
+```
+
+## License
+
+MIT