tests: test README.md

Thanks to
rust-lang/cargo#383 (comment)
for the assist.

Author: gadomski

README.md

@@ -19,10 +19,10 @@ println!("{}", object.id());
 
 Write STAC catalogs using the `BestPracticesRenderer`:
 
-```rust
+```rust,no_run
 use stac::{Stac, BestPracticesRenderer, Render, Writer, Write};
-let (stac, _) = Stac::read("data/catalog.json").unwrap();
-let renderer = BestPracticesRenderer::new("a/new/root/directory").unwrap();
+let (mut stac, _) = Stac::read("data/catalog.json").unwrap();
+let renderer = BestPracticesRenderer::new("a/new/root/directory");
 let writer = Writer::default();
 stac.write(&renderer, &writer).unwrap();
 ```

src/lib.rs

@@ -256,3 +256,17 @@ mod tests {
     }
     pub(crate) use roundtrip;
 }
+
+// From https://github.com/rust-lang/cargo/issues/383#issuecomment-720873790,
+// may they be forever blessed.
+#[cfg(doctest)]
+mod readme {
+    macro_rules! external_doc_test {
+        ($x:expr) => {
+            #[doc = $x]
+            extern "C" {}
+        };
+    }
+
+    external_doc_test!(include_str!("../README.md"));
+}