Update CONTRIBUTING.md (#589)

Add welcome paragraph and API documentation guidelines.
Mailing list discussion:
https://groups.google.com/a/tensorflow.org/forum/#!msg/swift/tQKFYL1Ykac/21eYYC3pBgAJ

Author: 8bitmp3

CONTRIBUTING.md

@@ -1,5 +1,14 @@
 # Contributing guidelines
 
+## Welcome!
+
+`swift-apis` is a carefully curated set of maintained APIs and functionality. 
+We generally aim to incubate new features in the library ecosystem that builds 
+on top of Swift for TensorFlow. Popular functionality, such as new layers or 
+helpful abstractions, often graduate into `swift-apis` after maturing in other 
+repositories first (e.g. `swift-models`), where the development team and the 
+community can try them out in context and iterate quickly.
+
 ## How to become a contributor and submit your own code
 
 ### Contributor License Agreements
@@ -59,6 +68,23 @@ Changes should conform to:
 
 With the exception that 4-space indendation be used.
 
+#### API documentation guidelines
+
+API documentation should follow guidelines from the
+["Write a documentation comment"](https://swift.org/documentation/api-design-guidelines/#write-doc-comment)
+section of the Swift API Design Guidelines:
+
+> *   **Use Swift’s
+>     [dialect of Markdown](https://developer.apple.com/library/archive/documentation/Xcode/Reference/xcode_markup_formatting_ref)**.
+> *   **Begin with a summary** that describes the entity being declared. Often,
+>     an API can be completely understood from its declaration and its summary.
+> *   **Optionally, continue** with one or more paragraphs and bullet items.
+>     Paragraphs are separated by blank lines and use complete sentences.
+
+For APIs ported from Python TensorFlow, use the API documentation from
+[tensorflow.org/api_docs](https://www.tensorflow.org/api_docs) (e.g. function
+parameter descriptions) as a starting point.
+
 ## Community
 
 It's a good idea to discuss any non-trivial submissions with the project