Refine wording in README for clarity

Author: andrehora

README.md

@@ -4,7 +4,7 @@
 
 Code evolution analysis for Git repositories.
 It currently supports Python, JavaScript, TypeScript, and Java.
-GitEvo can also be used to define custom code evolution metrics at the level of the concrete syntax tree (CST), thanks to [Tree-sitter](https://tree-sitter.github.io/tree-sitter).
+GitEvo can be used to define (custom) code evolution metrics at the level of the concrete syntax tree (CST), thanks to [Tree-sitter](https://tree-sitter.github.io/tree-sitter).
 
 Examples of reports: 
 [Flask](https://andrehora.github.io/gitevo-examples/python/flask.html),