Change README instruction to intall docpad@6.12 rather than 6.11

Following the README, I ran into some problems trying to get a local instance running:

```
error: Something went wrong with the action
error: An error occured: Object #<DocPad> has no method 'parseDocumentDirectory' TypeError: Object #<DocPad> has no method 'parseDocumentDirectory'
    at PartialsPlugin.module.exports.PartialsPlugin.populateCollections (/Users/miket/dev/forks/como-perder-peso/node_modules/docpad-plugin-partials/out/partials.plugin.js:100:14)
    at Object.balUtilFlow.fireWithOptionalCallback (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/flow.js:32:27)
    at BasePlugin.bindEvents (/usr/local/share/npm/lib/node_modules/docpad/out/lib/plugin.js:57:26)
    at Object.balUtilFlow.fireWithOptionalCallback (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/flow.js:32:27)
    at Object.EventEmitterEnhanced.emitSync (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/events.js:49:30)
    at Object.balUtilFlow.fireWithOptionalCallback (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/flow.js:32:27)
    at balUtilFlow.Group._Class.runTask.run (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/flow.js:377:30)
    at _Class.balUtilFlow.Group._Class.runTask (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/flow.js:382:11)
    at _Class.balUtilFlow.Group._Class.nextTask (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/flow.js:351:14)
    at _Class.balUtilFlow.Group._Class.complete (/usr/local/share/npm/lib/node_modules/docpad/node_modules/bal-util/out/lib/flow.js:270:18)
```
It turns out that `parseDocumentDirectory` was added in 6.12, so upping that version in README.

(this also solves an "outdated" error that refused to install the partials plugin. yay?) 
Unsure if there are any reasons to not just update to 6.22, but this was the smallest change to get it working locally.