docs: add build badge

Author: joscha

README.md

@@ -1,5 +1,7 @@
 # dependency-tree
 
+![build](https://github.com/Canva/dependency-tree-public/actions/workflows/node.js.yml/badge.svg)
+
 This package can create a dependency tree from a given set of files/folders.
 The nodes of the tree are files and the edges are file -> file dependencies.
 The most common way to express such a dependency between two files is some sort of import statement (`require()`, `import ... from`, `@import`) or a directive.