Add Append To A Keyword List as an elixir til

Add a new Elixir section too

elixir/append-to-a-keyword-list.md

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+# Append To A Keyword List
+
+If you have two keyword lists, you can append them like so:
+
+```elixir
+> a = [a: 1]
+[a: 1]
+> b = [b: 2]
+[b: 2]
+> a ++ b
+[a: 1, b: 2]
+```
+
+But what if something a bit more programmatic is happening and you are
+building up the additions to the keyword list based on variables?
+
+```elixir
+> x = :x
+:x
+> c = a ++ [x 5]
+** (CompileError) iex:5: undefined function x/1
+    (stdlib) lists.erl:1353: :lists.mapfoldl/3
+    (stdlib) lists.erl:1354: :lists.mapfoldl/3
+```
+
+That makes elixir think `x` is some function when in fact it is just a
+variable containing the keyword `:x`.
+
+Simply adding a comma doesn't quite do it either.
+
+```elixir
+> c = a ++ [x, 5]
+[{:a, 1}, :x, 5]
+```
+
+We need to wrap the internal part with curly braces to create the tuple that
+can then be appended to `a`.
+
+```elixir
+> c = a ++ [{x, 5}]
+[a: 1, x: 5]
+```