Add Convenience functions

docs/usage/convenience-functions.md

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+# Convenience functions
+
+OOXML and this library mainly uses a unit called twentieths of a point or `twip` for short. a twip is a typographical measurement, defined as 1/20 of a typographical point. One twip is 1/1440 inch, or 17.64 μm. This unit is not intuitive for many users, so some functions were created to help
+
+More info here: https://en.wikipedia.org/wiki/Twip
+
+## Convert Inches to Twip
+
+```ts
+import { convertInchesToTwip } from "docx";
+
+const twip = convertInchesToTwip(1); // returns 1440
+const twip = convertInchesToTwip(0.5); // returns 720
+```
+
+## Convert Millimeters to Twip
+
+```ts
+import { convertMillimetersToTwip } from "docx";
+
+const twip = convertMillimetersToTwip(50); // returns 2834
+```