Update the home page with major release changes 0.4 - 0.7

[tunetheweb]

@alanmcruickshank I think it would be good when we release 0.7.0 to update this:

sqlfluff/docs/source/index.rst

Lines 16 to 22 in c61003a

	* **0.1.x** involved a major re-write of the parser, completely changing
	the behaviour of the tool with respect to complex parsing.
	* **0.2.x** added templating support and a big restructure of rules
	and changed how users might interact with SQLFluff on templated code.
	* **0.3.x** drops support for python 2.7 and 3.4, and also reworks the
	handling of indentation linting in a potentially not backward
	compatible way.

Can anyone remember why 0.4.0 was not 0.3.x?
Similar for 0.5.0, and 0.6.0?
0.7.0 will be moving of DBT to plugin (not sure if worth mentioning retiring of exasol_fs or not?)

Would be nice to capture before our memories fade.

[alanmcruickshank]

Yeah:

• 0.4.0 dropped python 3.5, the dbt templater, source mapping and also introduced the python API
• 0.5.0 introduced some breaking changes to the API.
• 0.6.0 introduced parallel processing, which necessitated a big re-write of several innards
• 0.7.0 is dbt templater extraction and exasol_fs removal (I think we mention both).