Add Help For Non-Normal Mode Features as a vim til.

vim/help-for-non-normal-mode-features.md

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+# Help For Non-Normal Mode Features
+
+The majority of your time in vim will be spent in normal mode. You will
+often look to the documentation for help on any number of commands and
+bindings available in normal mode. For instance, to find out more about
+*goto file*, you may do `:h gf`. And if you want to read more about
+*yanking* lines of code, you may do `:h y`.
+
+But what about commands and bindings that aren't found in normal mode? What
+if you want to read about *yanking* text from visual mode? What if you want
+to get more details on insert's x-mode? Doing `:h y` and `:h ctrl-x`,
+respectively, won't do the trick because vim thinks you are talking about
+normal mode bindings.
+
+The docs for these and other non-normal mode features can be found by
+prepending `i_` and `v_` to the binding in question.
+
+To get at the docs for *yanking* from visual mode:
+
+```
+:h v_y
+```
+
+And to read up on insert's x-mode:
+
+```
+:h i_ctrl-x
+```