[docs] add missing rule to readme

Per 2839b5d#commitcomment-27350115

Author: ljharb

README.md

@@ -136,6 +136,7 @@ Enable the rules that you would like to use.
 ## JSX-specific rules
 
 * [react/jsx-boolean-value](docs/rules/jsx-boolean-value.md): Enforce boolean attributes notation in JSX (fixable)
+* [react/jsx-child-element-spacing](docs/rules/jsx-child-element-spacing.md): Enforce or disallow spaces inside of curly braces in JSX attributes and expressions.
 * [react/jsx-closing-bracket-location](docs/rules/jsx-closing-bracket-location.md): Validate closing bracket location in JSX (fixable)
 * [react/jsx-closing-tag-location](docs/rules/jsx-closing-tag-location.md): Validate closing tag location in JSX (fixable)
 * [react/jsx-curly-spacing](docs/rules/jsx-curly-spacing.md): Enforce or disallow spaces inside of curly braces in JSX attributes and expressions (fixable)