clarified needing to run generate_workspace from bazel source folder
clarified needing to run generate_workspace from bazel source folder

Hi,
If you feel this is unnecessary please feel free to close this PR but it took me some time and @lberki's help to understand that even though I built the binary from source and I have the source cloned then I still need to run the command from the source folder so I added that note in.

Closes #1489.

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Reviewed-on: https://github.com/bazelbuild/bazel/pull/1489
MOS_MIGRATED_REVID=127429574

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