doc: add guide to use the wire:"-" tag (#195)

docs/guide.md

@@ -300,6 +300,21 @@ func injectFooBar() FooBar {
 
 And similarly if the injector needed a `*FooBar`.
 
+It is sometimes useful to prevent certain fields from being filled in by the
+injector, especially when passing `*` to `wire.Struct`. You can tag a field with
+`` `wire:"-"` `` to have Wire ignore such fields. For example:
+
+```go
+type Foo struct {
+    mu sync.Mutex `wire:"-"`
+    Bar Bar
+}
+```
+
+When you provide the `Foo` type using `wire.Struct(new(Foo), "*")`, Wire will
+automatically omit the `mu` field. Additionally, it is an error to explicitly
+specify a prevented field as in `wire.Struct(new(Foo), "mu")`.
+
 ### Binding Values
 
 Occasionally, it is useful to bind a basic value (usually `nil`) to a type.