add information on testing and code style

fixes #2884

CONTRIBUTING.md

@@ -0,0 +1,54 @@
+Contributing
+============
+
+## Testing
+
+All features and bugs should have tests that verify the fix. You can run all
+tests using `npm test`.
+
+The most common type of test are tests that verify input and output of the
+Uglify transforms. These tests exist in `test/compress`. New tests can be added
+either to an existing file or in a new file `issue-xxx.js`.
+
+Tests that cannot be expressed as a simple AST can be found in `test/mocha`.
+
+## Code style
+
+- `LF` is always used as a line ending.
+- Statements end with semicolons.
+- Indentation uses 4 spaces, switch `case` 2 spaces.
+- Identifiers use `snake_case`.
+- Strings use double quotes (`"`).
+- The Uglify code only uses ES5, even in the `harmony` branch.
+- Line length should be at most 80 cols, except when it is easier to read a
+  longer line.
+- If both sides of a comparison are of the same type, `==` and `!=` are used.
+- Multiline conditions place `&&` and `||` first on the line.
+
+Example:
+
+```js
+if (compressor.option("comparisons")) switch (self.operator) {
+  case "==":
+  case "!=":
+    // void 0 == x => null == x
+    if (!is_strict_comparison && is_undefined(self.left, compressor)) {
+        self.left = make_node(AST_Null, self.left);
+    }
+    // "undefined" == typeof x => undefined === x
+    else if (compressor.option("typeofs")
+        && self.left instanceof AST_String
+        && self.left.value == "undefined"
+        && self.right instanceof AST_UnaryPrefix
+        && self.right.operator == "typeof") {
+        var expr = self.right.expression;
+        if (expr instanceof AST_SymbolRef ? expr.is_declared(compressor)
+            : !(expr instanceof AST_PropAccess && compressor.option("ie8"))) {
+            self.right = expr;
+            self.left = make_node(AST_Undefined, self.left).optimize(compressor);
+            if (self.operator.length == 2) self.operator += "=";
+        }
+    }
+    ...
+}
+```