enhance documentation around contributing to electron (#11887)

* add issues document

* add documentation coding style to doc

* copyediting

* replace `nodejs/node` with `electron/electron`
* fix commasplice
* fix two most important... s/is/are/
* omit unnecessary words

* add pull requests doc

* copyediting

* add general code style to styleguide

* updates to CONTRIBUTING.md

* copyediting

* mark shell blocks as ```sh
* mitigate phrase duplication e.g. 'best practice'
* lots of opinionated changes to omit unnecessary words

* fix numbering & re-apply changes that I overwrote

docs/development/coding-style.md

@@ -5,6 +5,23 @@ These are the style guidelines for coding in Electron.
 You can run `npm run lint` to show any style issues detected by `cpplint` and
 `eslint`.
 
+## General Code
+
+* End files with a newline.
+* Place requires in the following order:
+  * Built in Node Modules (such as `path`)
+  * Built in Electron Modules (such as `ipc`, `app`)
+  * Local Modules (using relative paths)
+* Place class properties in the following order:
+  * Class methods and properties (methods starting with a `@`)
+  * Instance methods and properties
+* Avoid platform-dependent code:
+  * Use `path.join()` to concatenate filenames.
+  * Use `os.tmpdir()` rather than `/tmp` when you need to reference the
+    temporary directory.
+* Using a plain `return` when returning explicitly at the end of a function.
+  * Not `return null`, `return undefined`, `null`, or `undefined`
+
 ## C++ and Python
 
 For C++ and Python, we follow Chromium's [Coding
@@ -21,6 +38,13 @@ document. The document mentions some special types, scoped types (that
 automatically release their memory when going out of scope), logging mechanisms
 etc.
 
+## Documentation
+
+* Write [remark](https://github.com/remarkjs/remark) markdown style
+
+You can run `npm run lint-docs` to ensure that your documentation changes are
+formatted correctly.
+
 ## JavaScript
 
 * Write [standard](https://npm.im/standard) JavaScript style.