fixed #12517 - added initial safety guide [skip ci]

Author: firewave

SAFETY.md

@@ -0,0 +1,28 @@
+# Cppcheck Safety Notes
+
+## Process Stability
+
+TODO: document thread vs process executor
+
+## Invoked Processes
+
+By default no additional external process will be invoked by the analysis.
+
+But there are some options which will utilize additional applications.
+
+`python` - Used for the execution of addons. Trigger by using the `--addons=` option and specifying addons in `cppcheck.cfg`.
+TODO: document lookup
+
+`clang-tidy` - Used for invoking an additional analysis via Clang-Tidy. Triggered by the `--clang-tidy` option.
+TODO: document lookup
+
+`clang` - Used to generate a CLang AST instead of an internal one. Triggered by the `--clang` option.
+TODO: document lookup
+
+## Output Files
+
+TODO
+
+## Required Permissions
+
+TODO