Add compile_error!() to the unsable book

Author: wesleywiser

src/doc/unstable-book/src/language-features/compile-error.md

@@ -0,0 +1,20 @@
+# `compile_error`
+
+The tracking issue for this feature is: [#40872]
+
+[#29599]: https://github.com/rust-lang/rust/issues/40872
+
+------------------------
+
+The `compile_error` feature adds a macro which will generate a compilation
+error with the specified error message.
+
+## Examples
+
+```rust
+#![feature(compile_error)]
+
+fn main() {
+    compile_error!("The error message"); //ERROR The error message
+}
+```