Add readme

README.md

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+derive(Error)
+=============
+
+[![Build Status](https://api.travis-ci.com/dtolnay/thiserror.svg?branch=master)](https://travis-ci.com/dtolnay/thiserror)
+[![Latest Version](https://img.shields.io/crates/v/thiserror.svg)](https://crates.io/crates/thiserror)
+[![Rust Documentation](https://img.shields.io/badge/api-rustdoc-blue.svg)](https://docs.rs/thiserror)
+
+This library provides a convenient derive macro for the standard library's
+[`std::error::Error`] trait.
+
+[`std::error::Error`]: https://doc.rust-lang.org/std/error/trait.Error.html
+
+```toml
+[dependencies]
+thiserror = "0.0"
+```
+
+*Compiler support: requires rustc 1.31+*
+
+<br>
+
+## Example
+
+```rust
+use thiserror::Error;
+
+#[derive(Error, Debug)]
+pub enum DataStoreError {
+    #[error("data store disconnected")]
+    Disconnect(#[source] io::Error),
+    #[error("the data for key `{0}` is not available")]
+    Redaction(String),
+    #[error("invalid header (expected {expected:?}, found {found:?})")]
+    InvalidHeader {
+        expected: String,
+        found: String,
+    },
+    #[error("unknown data store error")]
+    Unknown,
+}
+```
+
+<br>
+
+## Details
+
+- Errors may be enums, structs with named fields, tuple structs, or unit
+  structs.
+
+- A `Display` impl is generated for your error if you provide `#[error("...")]`
+  messages on the struct or each variant of your enum, as shown above in the
+  example.
+
+  The messages support a shorthand for interpolating fields from the error.
+
+    - `#[error("{var}")]` ⟶ `write!("{}", self.var)`
+    - `#[error("{0}")]` ⟶ `write!("{}", self.0)`
+    - `#[error("{var:?}")]` ⟶ `write!("{:?}", self.var)`
+    - `#[error("{0:?}")]` ⟶ `write!("{:?}", self.0)`
+
+  You may alternatively write out the full format args yourself, using arbitrary
+  expressions.
+
+  When providing your own format args, the shorthand does not kick in so you
+  need to specify `.var` in the argument list to refer to named fields and `.0`
+  to refer to tuple fields.
+
+  ```rust
+  pub enum Error {
+      #[error("invalid rdo_lookahead_frames {} (expected < {})", .0, i32::max_value())]
+      InvalidLookahead(i32),
+  }
+  ```
+
+- The Error trait's `source()` method is implemented to return whichever field
+  has a `#[source]` attribute, if any. This is for identifying the underlying
+  lower level error that caused your error.
+
+  Any error type that implements `std::error::Error` or dereferences to `dyn
+  std::error::Error` will work as a source.
+
+  ```rust
+  #[derive(Error, Debug)]
+  pub struct MyError {
+      msg: String,
+      #[source]
+      source: anyhow::Error,
+  }
+  ```
+
+- The Error trait's `backtrace()` method is implemented to return whichever
+  field has a type named `Backtrace`, if any.
+
+  ```rust
+  use std::backtrace::Backtrace;
+
+  #[derive(Error, Debug)]
+  pub struct MyError {
+      msg: String,
+      backtrace: Backtrace, // automatically detected
+  }
+  ```
+
+- See also the [`anyhow`] library for a convenient single error type to use in
+  application code.
+
+  [`anyhow`]: https://github.com/dtolnay/anyhow
+
+<br>
+
+#### License
+
+<sup>
+Licensed under either of <a href="LICENSE-APACHE">Apache License, Version
+2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option.
+</sup>
+
+<br>
+
+<sub>
+Unless you explicitly state otherwise, any contribution intentionally submitted
+for inclusion in this crate by you, as defined in the Apache-2.0 license, shall
+be dual licensed as above, without any additional terms or conditions.
+</sub>