Created SC1131 (markdown)

SC1131.md

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+## Use `elif` to start another branch.
+
+### Problematic code:
+
+```sh
+if false
+then
+  echo "hi"
+elseif true
+then
+  echo "ho"
+fi
+```
+
+### Correct code:
+
+```sh
+if false
+then
+  echo "hi"
+elif true
+then
+  echo "ho"
+fi
+```
+### Rationale:
+
+ShellCheck noticed that you appear to be using `elseif` or `elsif` as a keyword. This is not valid.
+
+`sh` instead uses `elif` as its alternative branch keyword.
+
+### Exceptions:
+
+If you have made your own function called `elseif` and intend to call it, you can ignore this message.
+
+### Related resources:
+
+* Help by adding links to BashFAQ, StackOverflow, man pages, POSIX, etc!