doc: add STYLE_GUIDE (moved from nodejs/docs)

Also add a reference in CONTRIBUTING.md to it.

CONTRIBUTING.md

@@ -85,6 +85,9 @@ Create a branch and start hacking:
 $ git checkout -b my-branch -t origin/master
 ```
 
+Any text you write should follow the [Style Guide](doc/STYLE_GUIDE.md),
+including comments and API documentation.
+
 ### Step 3: Commit
 
 Make sure git knows your name and email address:

doc/STYLE-GUIDE.md

@@ -0,0 +1,63 @@
+# Style Guide
+
+* Documents are written in markdown files.
+* Those files should be written in **`lowercase-with-dashes.md`.**
+  * Underscores in filenames are allowed only when they are present in the
+    topic the document will describe (e.g., `child_process`.)
+  * Filenames should be **lowercase**.
+  * Some files, such as top-level markdown files, are exceptions.
+  * Older files may use the `.markdown` extension. These may be ported to `.md`
+    at will. **Prefer `.md` for all new documents.**
+* Documents should be word-wrapped at 80 characters.
+* The formatting described in `.editorconfig` is preferred.
+  * A [plugin][] is available for some editors to automatically apply these rules.
+* Mechanical issues, like spelling and grammar, should be identified by tools,
+  insofar as is possible. If not caught by a tool, they should be pointed out by
+  human reviewers.
+* American English spelling is preferred. "Capitalize" vs. "Capitalise",
+  "color" vs. "colour", etc.
+* Though controversial, the [Oxford comma][] is preferred for clarity's sake.
+* Generally avoid personal pronouns in reference documentation ("I", "you",
+  "we".)
+  * Pronouns are acceptable in more colloquial documentation, like guides.
+  * Use **gender-neutral pronouns** and **mass nouns**. Non-comprehensive
+    examples:
+    * **OK**: "they", "their", "them", "folks", "people", "developers", "cats"
+    * **NOT OK**: "his", "hers", "him", "her", "guys", "dudes".
+* When combining wrapping elements (parentheses and quotes), terminal
+  punctuation should be placed:
+  * Inside the wrapping element if the wrapping element contains a complete
+    clause — a subject, verb, and an object.
+  * Outside of the wrapping element if the wrapping element contains only a
+    fragment of a clause.
+* Place end-of-sentence punctuation inside wrapping elements — periods go
+  inside parentheses and quotes, not after.
+* Documents must start with a level-one heading. An example document will be
+  linked here eventually.
+* Prefer affixing links to inlining links — prefer `[a link][]` to
+  `[a link](http://example.com)`.
+* When documenting APIs, note the version the API was introduced in at
+  the end of the section. If an API has been deprecated, also note the first
+  version that the API appeared deprecated in.
+* When using dashes, use emdashes ("—", Ctrl+Alt+"-" on OSX) surrounded by
+  spaces, per the New York Times usage.
+* Including assets:
+  * If you wish to add an illustration or full program, add it to the
+    appropriate sub-directory in the `assets/` dir.
+  * Link to it like so: `[Asset](/assets/{subdir}/{filename})` for file-based
+    assets, and `![Asset](/assets/{subdir}/{filename})` for image-based assets.
+  * For illustrations, prefer SVG to other assets. When SVG is not feasible,
+    please keep a close eye on the filesize of the asset you're introducing.
+* For code blocks:
+  * Use language aware fences. ("```js")
+  * Code need not be complete — treat code blocks as an illustration or aid to
+    your point, not as complete running programs. If a complete running program
+    is necessary, include it as an asset in `assets/code-examples` and link to
+    it.
+* When using underscores, asterisks and backticks please use proper escaping (**\\\_**, **\\\*** and **\\\`** instead of **\_**, **\*** and **\`**)
+* References to constructor functions should use PascalCase
+* References to constructor instances should be camelCased
+* References to methods should be used with parenthesis: `socket.end()` instead of `socket.end`
+
+[plugin]: http://editorconfig.org/#download
+[Oxford comma]: https://en.wikipedia.org/wiki/Serial_comma