Add style guideline about conditional conjunction formatting (#169)

Based on php/web-doc#33 by @tiffany-taylor

docs/style.md

@@ -41,14 +41,25 @@ CORRECT: The optional second parameter may be used to specify tags that should n
 Example: `<entry>5.2.11, 5.3.1</entry>`
 
 ## General Grammar
+
 The PHP Manual should be written with particular attention to general
 American English grammar and spelling.
-The [serial (Oxford) comma](https://en.wikipedia.org/wiki/Serial_comma)
+
+- The [serial (Oxford) comma](https://en.wikipedia.org/wiki/Serial_comma)
 should be used in a series of three or more terms.
-Contractions should be used appropriately.
-Special attention should be applied to sentence construction when using
+
+- Contractions should be used appropriately.
+
+- Special attention should be applied to sentence construction when using
 prepositions (i.e., sentences should not end in prepositions).
 
+- If a statement includes a conditional conjunction, the condition being
+met should come before the independent clause.
+The previous statement is an example of how a conditional conjuction
+should be formatted.
+See [PR#1565](https://github.com/php/doc-en/pull/1565) for another
+example.
+
 ## PHP Manual Terms
 
 There are various phrases and technical terms used throughout the manual where