[ docs ] Add documentation for DART_VM_OPTIONS

Change-Id: Icf66ff1f024c9f14bc8241b2b23f99f1ed8e0c45
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/367800
Reviewed-by: Derek Xu <derekx@google.com>
Commit-Queue: Ben Konyi <bkonyi@google.com>
Auto-Submit: Ben Konyi <bkonyi@google.com>
Commit-Queue: Derek Xu <derekx@google.com>

docs/Settings-VM-flags-in-standalone-executables.md

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+# Setting VM flags in standalone executables
+
+Dart VM flags and options can be provided to any executable generated using `dart compile exe` via the `DART_VM_OPTIONS` environment variable.
+
+`DART_VM_OPTIONS` should be set to a list of comma-separated flags and options with no whitespace. Options that allow for multiple values to be provided as comma-separated values are not supported (e.g., `--timeline-streams=Dart,GC,Compiler`).
+
+Example of a valid `DART_VM_OPTIONS` environment variable:
+
+    DART_VM_OPTIONS=--random_seed=42,--verbose_gc