Style guide: future tense

wiki/src/contribute/how/documentation/style_guide.mdwn

@@ -77,6 +77,16 @@
 
     - *Tails Installer* is available on Ubuntu 15.10 (Wily Werewolf) or later.
 
+- **future tense**
+
+Whenever possible, use present, not future, tense. Don't switch
+unnecessarily from present to future tense when present tense is
+sufficient to express a sequence of steps or events.
+
+Present tense is easier to read than past or future tense. Simple verbs
+are easier to read and understand than complex verbs, such as verbs in
+the progressive or perfect tense.
+
 <a id="digit_grouping"></a>
 
 - **digit grouping**