docs: create concepts/philosophy-of-uno (#1242)

doc/articles/concepts/overview/philosophy-of-uno.md

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+# Philosophy of Uno
+
+This document outlines the philosophy of Uno. It guides the development of past and future major architectural decisions.
+
+## Leverage existing tools
+
+We stand on the shoulders of giants, Microsoft's tooling is a treat to work with:
+
+- [Edit and Continue](https://docs.microsoft.com/en-us/visualstudio/debugger/edit-and-continue)
+- [Live Visual Tree](https://docs.microsoft.com/en-us/visualstudio/debugger/inspect-xaml-properties-while-debugging).
+- [XAML Hot Reload](https://docs.microsoft.com/en-us/visualstudio/debugger/xaml-hot-reload?view=vs-2019)
+
+The promise of the Uno Platform is to enable build your app with those tools and then deploy it to iOS, Android, and the web via WebAssembly. 
+
+## Create rich, responsive UIs
+
+Bland apps don't quite cut it these days. Strong support for animations, templating, and custom visual effects is a must. When phones come in all sizes and manufacturers are [gouging holes out of the screen area](https://www.cnet.com/pictures/phones-with-notches/), your app's layout had better be responsive. 
+
+## Let views do views
+
+Separation of model, view and presentation keeps your code loosely coupled and easy to maintain. Features like data binding and attached properties let you write clean, elegant MVVM-style code. 
+
+## Native intercompatibility (leave an escape hatch)
+
+100% code reuse is the ideal, but it must also be easy to access functionality specific to a single platform, or to incorporate native third-party libraries., and the Uno Platform must make as much as possible to not stand in the way.
+
+## Performance is a feature
+
+The slow antelope gets eaten, and the slow app gets 1-star ratings. We've done a lot of optimisation on the basis of profiling in real-world use cases, and we'll continue to do so. 

doc/articles/toc.yml

@@ -2,6 +2,8 @@
   items:
     - name: What is the Uno Platform?
       href: intro.md
+    - name: Philosophy of Uno
+      href: concepts/overview/philosophy-of-uno.md
     - name: API differences
       href: api-differences.md
     - name: Features list