doc: Add section on plurals to strings policy

Thanks to @pryds for explaining this to me on Transifex.

Author: laanwj

doc/translation_strings_policy.md

@@ -64,6 +64,44 @@ Avoid dividing up a message into fragments. Translators see every string separat
 There have been difficulties with use of HTML in translation strings; translators should not be able to accidentally affect the formatting of messages.
 This may sometimes be at conflict with the recommendation in the previous section.
 
+### Plurals
+
+Plurals can be complex in some languages. A quote from the gettext documentation:
+
+    In Polish we use e.g. plik (file) this way:
+    1 plik,
+    2,3,4 pliki,
+    5-21 pliko'w,
+    22-24 pliki,
+    25-31 pliko'w
+    and so on
+
+In Qt code use tr's third argument for optional plurality. For example:
+
+    tr("%n hour(s)","",secs/HOUR_IN_SECONDS);
+    tr("%n day(s)","",secs/DAY_IN_SECONDS);
+    tr("%n week(s)","",secs/WEEK_IN_SECONDS);
+
+This adds `<numerusform>`s to the respective `.ts` file, which can be translated separately depending on the language. In English, this is simply:
+
+    <message numerus="yes">
+        <source>%n active connection(s) to Bitcoin network</source>
+        <translation>
+            <numerusform>%n active connection to Bitcoin network</numerusform>
+            <numerusform>%n active connections to Bitcoin network</numerusform>
+        </translation>
+    </message>
+
+Where it is possible try to avoid embedding numbers into the flow of the string at all. e.g.
+
+    WARNING: check your network connection, %d blocks received in the last %d hours (%d expected)
+
+versus
+
+    WARNING: check your network connection, less blocks (%d) were received in the last %n hours than expected (%d).
+
+The second example reduces the number of pluralized words that translators have to handle from three to one, at no cost to comprehensibility of the sentence.
+
 ### String freezes
 
 During a string freeze (often before a major release), no translation strings are to be added, modified or removed.