Add a README.

README.md

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+# version\_check
+
+This tiny crate checks that the running or installed `rustc` meets some version
+requirements. The version is queried by calling the Rust compiler with
+`--version`. The path to the compiler is determined first via the `RUSTC`
+environment variable. If it is not set, then `rustc` is used. If that fails, no
+determination is made, and calls return `None`.
+
+## Usage
+
+Add to your `Cargo.toml` file:
+
+```toml
+[dependencies]
+version_check = "0.1"
+```
+
+## Examples
+
+Check that the running compiler is a nightly release:
+
+```rust
+extern crate version_check;
+
+match version_check::is_nightly() {
+    Some(true) => "running a nightly",
+    Some(false) => "not nightly",
+    None => "couldn't figure it out"
+};
+```
+
+Check that the running compiler is at least version `1.13.0`:
+
+```rust
+extern crate version_check;
+
+match version_check::is_min_version("1.13.0") {
+    Some((true, version)) => format!("Yes! It's: {}", version),
+    Some((false, version)) => format!("No! {} is too old!", version),
+    None => "couldn't figure it out".into()
+};
+```
+
+Check that the running compiler was released on or after `2016-12-18`:
+
+```rust
+extern crate version_check;
+
+match version_check::is_min_date("2016-12-18") {
+    Some((true, date)) => format!("Yes! It's: {}", date),
+    Some((false, date)) => format!("No! {} is too long ago!", date),
+    None => "couldn't figure it out".into()
+};
+```
+
+## Alternatives
+
+This crate is dead simple with no dependencies. If you need something more and
+don't care about panicking if the version cannot be obtained or adding
+dependencies, see [rustc_version](https://crates.io/crates/rustc_version).