Updated docs

Signed-off-by: Vishal Rana <vr@labstack.com>

Update type typo for Header handler (labstack#868)

The example for ServerHeader middleware is trying to set a `Echo/3.0` header,
but the response out put has a Echo/2.0 header, may confuse people.

issue: fixed labstack#755 (labstack#758)

Signed-off-by: Vishal Rana <vr@labstack.com>

updated docs

Signed-off-by: Vishal Rana <vr@labstack.com>

fixed labstack#729

Signed-off-by: Vishal Rana <vr@labstack.com>

updated migration guide

Signed-off-by: Vishal Rana <vr@labstack.com>

updated docs, readme and benchmark tests (labstack#718)

* updated docs, readme and benchmark tests

Signed-off-by: Vishal Rana <vr@labstack.com>

* fixed labstack#719

Signed-off-by: Vishal Rana <vr@labstack.com>

updated docs

Signed-off-by: Vishal Rana <vr@labstack.com>

updated docs

Signed-off-by: Vishal Rana <vr@labstack.com>

backport fix for labstack#623 from v3 (labstack#709)