docs: add docs for no-private-class-fields

docs/no-private-class-fields.md

@@ -0,0 +1,42 @@
+# no-private-class-fields
+
+This prevents the use of Private Class Fields
+
+```js
+class Foo {
+  static #bar = 1
+  #bar = 1
+
+  isFoo() {
+    return #bar === 1
+  }
+}
+```
+
+These will not be allowed because they are not supported in the following browsers:
+
+ - Edge (any version at the time of writing)
+ - Safari (any version at the time of writing)
+ - Firefox (any version at the time of writing)
+ - Chrome < 74
+
+
+## What is the Fix?
+
+Use of a WeakMap will cover most use cases:
+
+```js
+const fooPrivateState = new WeakMap()
+
+class Foo {
+  constructor() {
+    fooPrivateState.set(this, { bar: 1 })
+  }
+
+  isFoo() {
+    return (fooPrivateState.get(this) || {}).bar === 1
+  }
+}
+```
+
+This can be safely disabled if you intend to compile code with the `@babel/plugin-proposal-class-properties` Babel plugin.