Added mermaid-js to Diagramming Tools

[Prusdrum]

In one of the companies, I worked for we did a lot of integration with external services. It involved calling multiple APIs in different layers, i.e. mobile app calls our API, our API calls external service, etc. Mermaid allows creating a sequence diagram which worked best for documenting this kind of integration flows. It has its editor and declarative syntax, therefore we used to create these sequence diagrams and store the source file in mermaid syntax in the repository. For me it was a pretty nice solution, that's why I created this pull request.