Additional section on modifiers and padding, move traits details afte…

…r introduction for better flow (#10)

src/api/format-description.md

@@ -15,16 +15,7 @@
 # Format description
 
 A format description is the manner in which the time crate knows how a value should be formatted and
-parsed. However, a format description is not a single type; it is instead represented by two
-internal traits (one for formatting and one for parsing) that are implemented by a number of types.
-Currently, all types that implement one trait also implement the other, but this is not guaranteed.
-
-The following types currently implement both the `Formattable` and `Parsable` traits:
-
-- `FormatItem<'_>`
-- `[FormatItem<'_>]`
-- `T where <T as Deref>::Target: Formattable` (or `Parsable`)
-- All [well known formats](./well-known-format-descriptions.md)
+parsed.
 
 While it is possible to construct a format description manually, this is generally not recommended,
 as it is more tedious and less readable than the alternative. Unless you are doing this, you will
@@ -47,6 +38,27 @@ to have both consecutive literals and consecutive components. Components must be
 between brackets with optional whitespace. Escaping behavior varies by version, and is described
 below.
 
+## Modifiers
+
+Many of the components have optional modifiers to change how it is formatted or parsed.
+
+The `padding` modifier, such as used on the `hour` and `month` components can be padded
+with `space`, `zero` or `none`. In the case of `none` leading zeros are still accepted during
+parsing.
+
+## Traits
+
+A format description is not a single type; it is instead represented by two
+internal traits (one for formatting and one for parsing) that are implemented by a number of types.
+Currently, all types that implement one trait also implement the other, but this is not guaranteed.
+
+The following types currently implement both the `Formattable` and `Parsable` traits:
+
+- `FormatItem<'_>`
+- `[FormatItem<'_>]`
+- `T where <T as Deref>::Target: Formattable` (or `Parsable`)
+- All [well known formats](./well-known-format-descriptions.md)
+
 ## Versioning
 
 There are multiple versions of the format description syntax in `time`. Similar to Rust editions,