Add const unless it can't be

Cpp/const-unless-it-cant-be.md

+`const` unless it can't be
+==========================
+
+Everything should always be const unless it can't be. This includes both function
+parameters as well as functions themselves. This improves readability and self-
+documentation of the code. It will also save a lot of headaches and frustrations
+when debugging the program(s).
+
+> I am a full const nazi nowadays, and I chide any programmer that doesn’t const
+> every variable and parameter that can be. - [John Carmack, 2013](http://kotaku.com/454293019)
+
+On that note, always pass objects as const reference when possible. All of these
+would help greatly towards inference of what a function does.
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