Update external_docs.rst

Added "Pootle Tutorial: Guide for translators and developers" for Pootle 2.5.1
Roadmap: Update roadmap to match 2.6.0 (FIXME)
Release 2.6.0-rc1
Update translations
Update LINGUAS
release: add release note credits
docs: Add Github step to release process
docs: Do not update the dashboard for release candidates
Accounts: Fix a lingering bad url reference
Accounts: Fix a missing import
Accounts: Link to the correct password change view
Accounts: Fix the missing user_form.html template
Navbar: Use a clearer orange link for ADMIN
Accounts: More complete Profile->User migration
requirements: Require South 1.0 until we move to Django 1.7
Accounts: Do not run constraint-related migrations on sqlite3

See SOUTH-775: http://south.aeracode.org/ticket/775
Store: Fix a NameError on vcs_update
Docs: Improve the release notes index page

Moved the upgrade notices from changelog to release notes index, included
a link back to changelog from release notes index, and changed to
link to TTK release notes instead of changelog.
Docs: Move 2.1.1 changelog to release notes

This way all the releases that can be directly upgraded to latest release
now have release notes, that are much more detailed than previous changelog.

The release notes were retrieved from SourceForge.
Docs: Add more detailed upgrade instructions