Add declaration before code to guidelines

Declarations before code has been informally supported and we have
several examples in the code. This commit adds that guideline to the
coding guidelines.

docs/StyleGuide.md

@@ -37,6 +37,21 @@ elaborates on the situation. Instead, we've tried our best to develop
 a consistent style that roughly follows the style of the PostgreSQL
 source code.
 
+### Declarations before code
+
+C99 supports having code before a declaration, similar to C++, and we
+also support this in the code. For instance, the following code
+follows the guidelines:
+
+```C
+void some_function()
+{
+    prepare();
+	int result = fetch();
+	...
+}
+```
+
 ### Function and variable names
 
 For clarity and consistency, we've chosen to go with lowercase