Adding an example to the docs for how to implement a custom entity data service

[sstorie]

Adding an example for how to implement a custom entity data service

Purpose

This PR adds a simple example for how to create and register custom entity data services, which will be common in scenarios where people was to override the default behavior when retrieving and saving data.

Does this introduce a breaking change?

[ ] Yes
[X] No

Pull Request Type

What kind of change does this Pull Request introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[X] Documentation content changes
[ ] Other... Please describe:

How to Test

• Get the code

git clone [repo-address]
cd [repo-name]
git checkout [branch-name]
npm install

• Test the code

What to Check

Verify that the following are valid

• ...

Other Information

This is a result of the discussion in https://github.com/johnpapa/angular-ngrx-data/issues/143. It also seems that Prettier made some small updates to whitespace in certain parts of the document. I didn't change anything except to add the Further customization section at the end.

[wardbell]

Thank you! Merging. Then following up with sample update that essentially follows your example. I will tweak your prose to match the sample.