Update diagrams section and add Mermaids example (nhs-england-tools#165)

<!-- markdownlint-disable-next-line first-line-heading -->
## Description

Small update to the README with suggestions for diagram options that
keep the source under version control. I.e. `drawio` which is already
used in the examples, but also `mermaids`. Also provide a basic
`mermaids` example.

## Context

Updates guidance on diagram tooling.

## Type of changes

<!-- What types of changes does your code introduce? Put an `x` in all
the boxes that apply. -->

- [ ] Refactoring (non-breaking change)
- [ ] New feature (non-breaking change which adds functionality)
- [ ] Breaking change (fix or feature that would change existing
functionality)
- [ ] Bug fix (non-breaking change which fixes an issue)

## Checklist

<!-- Go over all the following points, and put an `x` in all the boxes
that apply. -->

- [ ] I am familiar with the [contributing
guidelines](../docs/CONTRIBUTING.md)
- [ ] I have followed the code style of the project
- [ ] I have added tests to cover my changes
- [ ] I have updated the documentation accordingly
- [ ] This PR is a result of pair or mob programming

---

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Author: fstewart-nhs

README.md

@@ -91,6 +91,18 @@ The [C4 model](https://c4model.com/) is a simple and intuitive way to create sof
 
 ![Repository Template](./docs/diagrams/Repository_Template_GitHub_Generic.png)
 
+The source for diagrams should be in Git for change control and review purposes. Recommendations are [draw.io](https://app.diagrams.net/) (example above in [docs](.docs/diagrams/) folder) and [Mermaids](https://github.com/mermaid-js/mermaid). Here is an example Mermaids sequence diagram:
+
+```mermaid
+sequenceDiagram
+    User->>+Service: GET /users?params=...
+    Service->>Service: auth request
+    Service->>Database: get all users
+    Database-->>Service: list of users
+    Service->>Service: filter users
+    Service-->>-User: list[User]
+```
+
 ### Modularity
 
 Most of the projects are built with customisability and extendability in mind. At a minimum, this can be achieved by implementing service level configuration options and settings. The intention of this section is to show how this can be used. If the system processes data, you could mention here for example how the input is prepared for testing - anonymised, synthetic or live data.