New section: Omit zero value fields

Add new section under Initializing Structs that explains that advises
readers to not specify values of fields with zero values per uber-go#109.

Based on discussion in the issue, this adds an exception for cases where
the field name would provide significant context, like test cases.

style.md

@@ -98,6 +98,7 @@ row before the </tbody></table> line.
   - [Use Raw String Literals to Avoid Escaping](#use-raw-string-literals-to-avoid-escaping)
   - [Initializing Structs](#initializing-structs)
       - [Use Field Names to Initialize Structs](#use-field-names-to-initialize-structs)
+      - [Omit Zero Value Fields in Structs](#omit-zero-value-fields-in-structs)
       - [Initializing Struct References](#initializing-struct-references)
   - [Initializing Maps](#initializing-maps)
   - [Format Strings outside Printf](#format-strings-outside-printf)
@@ -2837,6 +2838,54 @@ tests := []struct{
 }
 ```
 
+#### Omit Zero Value Fields in Structs
+
+When initializing structs with field names, omit fields that have zero values.
+These values will be set by Go automatically.
+
+<table>
+<thead><tr><th>Bad</th><th>Good</th></tr></thead>
+<tbody>
+<tr><td>
+
+```go
+user := User{
+  FirstName: "John",
+  LastName: "Doe",
+  MiddleName: "",
+  Admin: false,
+}
+```
+
+</td><td>
+
+```go
+user := User{
+  FirstName: "John",
+  LastName: "Doe",
+}
+```
+
+</td></tr>
+</tbody></table>
+
+This helps reduce noise for readers by omitting values that are default in
+that context. Only meaningful values are specified.
+
+Exception: Even with a zero value, field names sometimes provide valuable
+context for readers. In particular, test cases in [Test Tables](#test-tables)
+can benefit from names of fields even when they are zero-valued.
+
+```go
+tests := []struct{
+  give string
+  want int
+}{
+  {give: "0", want: 0},
+  // ...
+}
+```
+
 #### Initializing Struct References
 
 Use `&T{}` instead of `new(T)` when initializing struct references so that it