Add README to gb plugin

plugins/getgb/README.md

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+# *gb* plugin
+
+- aims to support completion for all *gb* commands
+- also supports completion for *gb* plugins such as *vendor*
+
+See http://getgb.io for the full *gb* documentation
+
+## Caveats
+
+- The *git* plugin installs an alias *gb* that usually conflicts with the *gb* program