Created SC1090 (markdown)

SC1090.md

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+## This source will be skipped since it's not constant.
+
+### Problematic code:
+
+    source "$1/lib.sh"
+
+### Correct code:
+
+    # shellcheck disable=SC1090
+    source "$1/lib.sh"
+
+### Rationale:
+
+ShellCheck is not able to include sourced files from non-constant paths.
+
+Instead, be aware that anything in the sourced file will be ignored, and then disable the message.
+
+### Exceptions:
+
+Ignore the message with a [[directive]].