Created SC2153 (markdown)

SC2153.md

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+## Possible Misspelling: MYVARIABLE may not be assigned, but MY_VARIABLE is.
+
+### Problematic code:
+
+    MY_VARIABLE="hello world"
+    echo "$MYVARIABLE"
+
+### Correct code:
+
+    MY_VARIABLE="hello world"
+    echo "$MY_VARIABLE"
+
+### Rationale:
+
+ShellCheck has noticed that you reference a variable that is not assigned in the script, which has a name remarkably similar to one that is explicitly assigned. You should verify that the variable name is spelled correctly.
+
+Note: This error only triggers for environment variables (all uppercase variables), and only when they have names similar to something assigned in the script. If the variable is script-local, it should by convention have a lowercase name, and will in that case be caught by [SC2154] whether or not it resembles another name.
+
+### Exceptions:
+
+If you've double checked and ensured that you did not intend to reference the specified variable, you can disable this message with a [[directive]]. The message will also not appear for guarded references like `${ENVVAR:-default}` or `${ENVVAR:?Unset error message here}`.