Add substitution key documentation (#41)

Author: de-sh

docs/_sidebar.md

@@ -21,6 +21,7 @@
 	* [Lowercase Key](checks/lowercase_key.md)
 	* [Quote Character](checks/quote_character.md)
 	* [Space Character](checks/space_character.md)
+	* [Substitution Key](checks/substitution_key.md)
 	* [Trailing whitespace](checks/trailing_whitespace.md)
 	* [Unordered Key](checks/unordered_key.md)
 

docs/checks/substitution_key.md

@@ -0,0 +1,13 @@
+# Substitution Key
+
+Detects if a substitution is not properly performed on key assignment:
+
+```env
+❌ Wrong
+ABC=${BAR
+FOO="$BAR}"
+
+✅ Correct
+ABC=${BAR}
+FOO="$BAR"
+```