add documentation on positioning units

See further discussions in #50, #191.

docs/usage/document.md

@@ -5,7 +5,7 @@
 To create a new document, it is very easy:
 
 ```js
-var doc = new docx.Document();
+const doc = new docx.Document();
 ```
 
 ## Document properties
@@ -13,7 +13,7 @@ var doc = new docx.Document();
 You can add properties to the Word document by specifying options, for example:
 
 ```js
-var doc = new docx.Document({
+const doc = new docx.Document({
     creator: "Dolan Miu",
     description: "My extremely interesting document",
     title: "My Document",
@@ -22,14 +22,18 @@ var doc = new docx.Document({
 
 ### Full list of options:
 
-```
-creator
-description
-title
-subject
-keywords
-lastModifiedBy
-revision
-```
+
+* creator
+* description
+* title
+* subject
+* keywords
+* lastModifiedBy
+* revision
 
 You can mix and match whatever properties you want, or provide no properties.
+
+### units for positioning
+
+Various parts of the API require positioning arguments. The units are "20ths of a point" from the [OOXML](http://officeopenxml.com/index.php) specification.
+See [Lars Corneliussen's blog post](https://startbigthinksmall.wordpress.com/2010/01/04/points-inches-and-emus-measuring-units-in-office-open-xml/) for more information and how to convert units.