Prep for 7.0.0-alpha9.

Merge pull request drush-ops#1097 from colans/zsh-autocompletion

Explain how to set up ZSH autocompletion in the docs

Prep for alpha7.

Prep for drush_version=7.0.0-alpha6

Prep for 6.5.0

Prep for alpha5 release.

Prep for 7.0.0-alpha4

Prep for 6.4.0

Prepare for alpha3

7.0.0-alpha2 tag