Added first part of the documentation

docs/variable_terminals.md

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+# Variable Terminals
+
+Variable terminals are pieces of data which tell the type of data a token should have
+but don't explictly tell the exact content of a token.
+
+As an example, this could mean that a token must be alphanumeric, like a variable identifier,
+but the name doesn't really matter.
+
+## List of Variable Terminals
+
+- word
+- word_with_underscored
+- keyword
+- operator
+- special
+- number
+- number_with_underscores
+- alphanumeric
+- alphanumeric_with_underscores
+
+TODO: More descriptive content