**Description of the change**

A documentation-only change making it clear which fields in the template
`Secret` are directly copied, which undergo processing, and which are
ignored.

This addresses the confusion in issue #1445.

**Benefits**

Clarity for users experimenting with the Secret templating options. The
idea that `.spec.template.data` can be used is apparent as it is
generated by `kubeseal` (defaulting to `null`), but the README does not
make the usage or use-case clear.

**Possible drawbacks**

This change highlights increases the amount of technical detail in the
README that is not relevant to most users.
It also increases the attention drawn to the data templating feature
which could cause requests for changes in that code's scope.

**Alternatives**

Implementation details on the templating logic could be moved into their
own file within `docs/`.

**Applicable issues**

- fixes #1445

---------

Signed-off-by: Benedict Harcourt <ben.harcourt@harcourtprogramming.co.uk>
Signed-off-by: Alejandro Moreno <amorenoc@vmware.com>
Co-authored-by: Alejandro Moreno <amorenoc@vmware.com>