Add readme.md to plugins

A table can be done at https://github.com/bitshares/gwallet/tree/master/plugins

And a README.md inside each plugin with some text, screenshots, etc of the commands plugin supports.

Format should be similar on each plugin and it should be defined.

Review the plugins directory linked in the issue and identify each plugin and the commands it supports. Define a consistent README.md format, add a README.md inside each plugin with text and screenshots where available, and add a table covering the plugins; completion means every plugin follows the agreed format.

Written by the indexing model from the issue text.