Merge pull request #17 from input-output-hk/newhoggy/CLI-era-command-…

…groups-migration

CLI era command groups migration

docs/CLI-era-command-groups.md

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+`cardano-cli` is transitioning to a new command structure where instead of each
+era-senstivie command getting an era flag (eg `--babbage-era`, `--conway-era`),
+we will have the era as a top-level command.
+
+For the moment, this will be:
+
+* `byron`: already exists and will be unchanged
+* `shelley`: re-purposed to mean `shelley-era` only rather than `shelley-based` era
+* `allegra`: `allegra` era only
+* `mary`: `mary` era only
+* `alonzo`: `alonzo` era only
+* `babbage`: `babbage` era only
+* `conway`: `conway` era only
+
+Then we will have the following:
+
+* `latest`: the latest supported era.  The latest officially supported era in the CLI
+* `experimental`: the era following the latest era that is not yet officially supported in the CLI
+* `legacy`: the commands that existed before the restructuring.  Gradually these commands
+  will be migrated to fit into the new command structure.
+
+The `latest` command group is implied, so any sub-commands there-in will also be available
+at the top-level for convenience.
+
+The `legacy` command group is also implied, so any sub-commands there-in will be
+available at the top-level for convenience provided that the `latest` version of that
+command does not yet exist.