docs: add documentation about new clang-format formatter

Document how to use the `clang-format` formatter added to this
repository in #372. Include instructions for formatting files on save
in VS Code, IntelliJ IDEs, and Vim.

Add a VS Code workspace setting configuration for enabling automatic
source code formatting with the `clang-format` VS Code extension.

Add `.vimrc` and `.nvimrc` so Vim and Neovim configurations of the
formatter are local-only.

Part of #373.

Author: ayazhafiz

.gitignore

@@ -9,3 +9,5 @@ integration/out
 dist/
 **/.idea/
 **/tsconfig.tsbuildinfo
+.vimrc
+.nvimrc

.vscode/settings.json

@@ -2,5 +2,11 @@
   "editor.insertSpaces": true,
   "tslint.enable": true,
   "typescript.tsc.autoDetect": "off",
-  "typescript.preferences.quoteStyle": "single"
+  "typescript.preferences.quoteStyle": "single",
+  "//format-on-save-comment": "Format ts files on save with `clang-format.executable`. If `clang-format.executable` is not being used, these two settings should be removed otherwise it will break existing formatting. You can instead run `yarn format` to manually format your code.",
+  "[typescript]": {
+    "editor.formatOnSave": true
+  },
+  "//clang-format-comment": "Please install https://marketplace.visualstudio.com/items?itemName=xaver.clang-format to take advantage of `clang-format` in VSCode. (See https://clang.llvm.org/docs/ClangFormat.html for more info `clang-format`.)",
+  "clang-format.executable": "${workspaceRoot}/node_modules/.bin/clang-format"
 }

DEVELOPER.md

@@ -0,0 +1,33 @@
+## Formatting source code
+
+This repository uses the [NPM distribution](https://www.npmjs.com/package/clang-format) of
+[`clang-format`](http://clang.llvm.org/docs/ClangFormat.html) to format source code.
+
+Code is automatically formatted by running `yarn format`. You can also set up your IDE to format
+files on each save.
+
+### VS Code
+
+1. Install the
+[`Clang-Format` extension](https://marketplace.visualstudio.com/items?itemName=xaver.clang-format)
+for VS Code.
+
+VS Code will automatically pick up relevant settings from this repository's
+[`.vscode/settings.json`](./.vscode/settings.json).
+
+### WebStorm / IntelliJ
+
+1. Install the [`ClangFormatIJ`](https://plugins.jetbrains.com/plugin/8396-clangformatij) plugin for
+   IntelliJ IDEs.
+2. Open `Preferences->Tools->clang-format`.
+3. Set the field named "PATH" to `<PATH_TO_REPOSITORY>/node_modules/.bin/`.
+
+### Vim
+
+1. Install [Vim Clang-Format](https://github.com/rhysd/vim-clang-format).
+2. Create a [project-specific `.vimrc`](https://andrew.stwrt.ca/posts/project-specific-vimrc/) in
+   the repository root directory containing
+
+```vim
+let g:clang_format#command = '<PATH_TO_REPOSITORY>/node_modules/.bin/clang-format'
+```