[coreboot-gerrit] Change in coreboot[master]: Documentation: Rename submodules.txt to submodules.md

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Change subject: Documentation: Rename submodules.txt to submodules.md
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Patch Set 1: Verified+1

Build Successful

https://qa.coreboot.org/job/coreboot-gerrit/69741/ : SUCCESS

https://qa.coreboot.org/job/coreboot-checkpatch/24039/ : SUCCESS