document write! in the book

book/src/format.md

@@ -1,6 +1,8 @@
-# `#[derive(Format)]`
+# implementing `Format`
 
-To implement the `Format` trait for a struct or enum use the `derive` attribute.
+## `#[derive(Format)]`
+
+The easiest way to implement the `Format` trait for a struct or enum is to use the `derive` attribute.
 
 ``` rust
 # extern crate defmt;
@@ -24,5 +26,45 @@ enum Request {
 
 NOTE: Like built-in derives like `#[derive(Debug)]`, `#[derive(Format)]` will add `Format` bounds to the generic type parameters of the struct.
 
-NOTE: At the moment is not possible to implement the `Format` trait manually. Do *not* use the API
-used by the expansion of the `derive(Format)` macro; it is currently *unstable*.
+NOTE: Do *not* use the API used by the expansion of the `derive(Format)` macro; it is *unstable*.
+
+## `write!`
+
+It is also possible to implement the `Format` trait manually.
+This trait has a single required method: `format`.
+In this method you need to format the value (`self`) into the given `Formatter` argument using the `defmt::write!` macro.
+Example below:
+
+``` rust
+# extern crate defmt;
+struct S {
+    x: u8,
+    y: u16,
+}
+
+impl defmt::Format for S {
+    fn format(&self, f: &mut defmt::Formatter) {
+        defmt::write!(f, "S {{ x: {:u8}, y: {:u16} }}", self.x, self.y)
+    }
+}
+```
+
+NOTE: in defmt v0.1.x the `write!` macro must be invoked at most once within the implementation of the `fmt` method. Invoking the method more than once will produce a panic
+
+## newtypes
+
+If you need to implement `Format` for some "newtype" struct you can delegate the formatting to the inner type.
+Example below:
+
+``` rust
+# extern crate defmt;
+struct MyU8 { inner: u8 }
+
+impl defmt::Format for MyU8 {
+    fn format(&self, f: &mut defmt::Formatter) {
+        self.inner.format(f)
+    }
+}
+```
+
+**WARNING** never call `format` more than once! that corrupts the data stream