mercurial: add hga alias to README (#7547)

plugins/mercurial/README.md

@@ -26,6 +26,7 @@ Update .zshrc:
 ### What's inside?
 #### Adds handy aliases:
 ###### general
+* `hga` - 'hg add`
 * `hgc` - `hg commit`
 * `hgb` - `hg branch`
 * `hgba` - `hg branches`