Added contributing guide for docs

docs/api/CONTRIBUTING.md

+Contributing to the documentation
+=================================
+
+- To link to the page for `ClassName`, use `[page:ClassName link title]` (or just `[page:ClassName]`). Use `[page:ClassName.memberName]` to link to a class member (a property or method) called `memberName` on the page for `ClassName`. You can write `[page:.memberName]` to link to a property or method called `memberName` on the same page.
+- Use `[example:exampleName title]` (ot just `[example:exampleName]`) to link to the example `threejs.org/examples/#exampleName`.
+- Document a property by writing `<h3>[property:TypeName propertyName]</h3>`/
+- Document a method using `<h3>[method:ReturnType methodName]</h3>`.
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