docs: add docs for no-optional-chaining

docs/no-optional-chaining.md

@@ -0,0 +1,38 @@
+# no-optional-chaining
+
+This prevents the use of Optional Chaining:
+
+```js
+const baz = obj?.foo?.bar?.baz; // 42
+```
+
+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 (any version at the time of writing)
+
+## What is the Fix?
+
+If the expression is short, you can consider using a ternary operator:
+
+```js
+// these are equivalent:
+foo?.bar
+foo == null ? void 0 : foo.bar;
+```
+
+You can also use the Logical OR operator to avoid throwing, although these can look messy:
+
+```js
+const baz = (((obj || {}).foo || {}).bar || {}).baz
+```
+
+Lastly, you could consider using a utility function such as [lodash' `get`](https://lodash.com/docs/4.17.15#get)
+
+```js
+const baz = _.get(obj, 'foo.bar.baz')
+```
+
+This can be safely disabled if you intend to compile code with the `@babel/plugin-proposal-optional-chaining` Babel plugin.