Implement virtual keyword

docs/060-breaking-changes.rst

@@ -60,6 +60,8 @@ This section highlights changes that affect syntax and semantics.
   which send value will revert. If you only implement the receive and not the fallback function, calling a non-existing function on your contract in error is not possible anymore. Unless you are following an upgrade or proxy
   pattern, you should not need to implement the fallback function.
 
+* Functions can now only be overridden when they are either marked with the ``virtual`` keyword or defined in an interface. When overriding a function or modifier, the new keyword ``override`` must be used. When overriding a function or modifier defined in multiple parallel bases, all bases must be listed in parentheses after the keyword like so: ``override(Base1, Base2)``.
+
 
 How to update your code
 =======================
@@ -81,6 +83,8 @@ This section gives detailed instructions on how to update prior code for every b
 
 * Choose unique identifiers for variable declarations in inline assembly that do not conflict with declartions outside the inline assembly block.
 
+* Add ``virtual`` to every non-interface function you intend to override.  For single inheritance, add ``override`` to every overriding function. For multiple inheritance, add ``override(A, B, ..)``, where you list all contracts that define the overridden function in the brackets. When multiple bases define the same function, the inheriting contract must override all conflicting functions.
+
 New Features
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