Add link to E.12 in refactorings page

This one was listed, but not linked

Author: herwinw

learning/REFACTORINGS.md

@@ -5,6 +5,7 @@
 * [ES.22: Don’t declare a variable until you have a value to initialize it with](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#es22-dont-declare-a-variable-until-you-have-a-value-to-initialize-it-with)
 * [ES.30: Don’t use macros for program text manipulation](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#es30-dont-use-macros-for-program-text-manipulation)
 * [ES.49: If you must use a cast, use a named cast](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#es49-if-you-must-use-a-cast-use-a-named-cast)
+* [E.12: Use noexcept when exiting a function because of a throw is impossible or unacceptable](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#e12-use-noexcept-when-exiting-a-function-because-of-a-throw-is-impossible-or-unacceptable)
 * [Con.4: Use const to define objects with values that do not change after construction](https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#con4-use-const-to-define-objects-with-values-that-do-not-change-after-construction)
 
 | Rule  |   Core Guideline   | Count |