docs: add docs for no-do-expression

docs/no-do-expression.md

@@ -0,0 +1,49 @@
+# no-do-expression
+
+This prevents the use of Do Expressions
+
+```js
+let a = do {
+  if(x > 10) {
+    'big'
+  } else {
+    'small'
+  }
+}
+```
+
+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:
+let a = do {
+  if(x > 10) {
+    'big'
+  } else {
+    'small'
+  }
+}
+let a = x > 10 ? 'big' : 'small'
+```
+
+You can also avoid the use of the do expression, and declare a variable upfront without assigning it immediately:
+
+```js
+let a
+if(x > 10) {
+  let a = 'big'
+} else {
+  let a = 'small'
+}
+```
+
+This can be safely disabled if you intend to compile code with the `@babel/plugin-proposal-do-expressions` Babel plugin.