Suggest configuration improvements to user

**Describe the problem/challenge you have**
This issue originates from issue #278. There are scenarios where configuration (schema, data values, templates, etc) may be valid and running `ytt` succeeds, but the configuration could be improved or simplified.

**Describe the solution you'd like**
This issue is a place to collect **scenarios** where `ytt` could alert the user in some way of an improved experience.
Please add your scenarios here!
Note: this is not necessarily the place to discuss __how__ `ytt` would do the alert. There is some discussion in this issue on that #278.

**Anything else you would like to add:**
❗ This is a list of some scenarios identified so far:❗

**Scenario** Duplicated data values
**The improvement is** You may remove one of the data values since it is redundant.

schema.yml
```
#@schema/definition name="data/values"
foo: "apple"
```
datavalues.yml
```
#@data/values
foo: "apple"
```
---

**Scenario** Configuration Author provides a default value in a schema, but recommends it is changed
**The improvement is** message saying this value is not recommended for usage

For example: if a Configuration Author provides a default value in a schema to be used in Kubernetes that has:
```
spec:
replicas: 1
```
But 1 replica is not appropriate for that resource and it will not behave as intended if used without increasing the number.

---

**Scenario** Unused data values
**The improvement is** A data value that is defined and not used can be removed. It also may indicate that it was meant to be used, but accidentally hardcoded in a template.

---
**Scenario** Remove `#@overlay/append`
**The improvement is** Let the user know they may remove this annotation since it is the default behavior (v0.32.0+)