Add README.md

Author: jonenst

README.md

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+# Gridsuite Aggregator
+
+Inital clone
+```
+$ git clone --recurse-submodules --remote-submodules --jobs 8 https://github.com/gridsuite/aggregator
+$ git submodule foreach --recursive 'git checkout main'
+```
+
+Update all
+```
+# --rebase or --merge as you wish
+$ git submodule update --remote --recursive --rebase --jobs 8
+```