Change the default value of critical to false.

This means all package groups are non-critical by default.
Update documentation accordingly.

src/modules/netinstall/NetInstallPage.h

@@ -38,11 +38,11 @@ class Page_NetInst;
 struct Group
 {
     Group()
-        : Group( "","",false, false, true ) { }
+        : Group( "","",false, false, false ) { }
     Group( QString name, QString description, bool selected, bool hidden, bool critical )
         : name( name ), description( description ), selected( selected ), hidden( hidden ), critical( critical ) { }
     Group( QString name, QString description )
-        : Group( name, description, false, false, true ) { }
+        : Group( name, description, false, false, false ) { }
 
     QString name;
     QString description;
@@ -51,7 +51,7 @@ struct Group
     // See README.md for a description of these fields.
     bool selected = false;
     bool hidden = false;
-    bool critical = true;
+    bool critical = false;
 };
 
 class NetInstallPage : public QWidget

src/modules/netinstall/README.md

@@ -33,7 +33,7 @@ Three more keys are supported:
       - selected: if true, display the group as selected. Defaults to false.
       - critical: if true, make the installation process fail if installing
         any of the packages in the group fails. Otherwise, just log a warning.
-        Defaults to true.
+        Defaults to false.
 
 If you set both *hidden* and *selected* for a group, you are basically creating a "default" group of packages
 which will always be installed in the user's system.