Merge pull request #13834 from vector-im/t3chguy/docs

Add app-load doc

docs/app-load.md

@@ -0,0 +1,81 @@
+# App load order
+
+Old slow flow:
+![image](https://user-images.githubusercontent.com/2403652/73848963-00a2a080-4821-11ea-97d4-1200fc2638f3.png)
+
+Current more parallel flow:
+![image](https://user-images.githubusercontent.com/2403652/83146440-303a2900-a0ee-11ea-806b-4f53f039b957.png)
+
+<details><summary>Code</summary>
+<p>
+<pre><code>
+digraph G {
+  node [shape=box];
+
+  subgraph cluster_0 {
+    color=orange;
+    node [style=filled];
+    label = "index.ts";
+
+    entrypoint, s0, ready [shape=point];
+    rageshake, config, i18n, theme, skin, olm [shape=parallelogram];
+    mobile [shape=diamond, label="mobile"];
+    modernizr [shape=diamond];
+    redirect, incompatible [shape=egg];
+
+    entrypoint -> rageshake;
+    rageshake -> mobile [color=blue];
+    mobile -> s0 [label="No"];
+    mobile -> redirect [label="Yes"];
+
+    s0 -> platform;
+    s0 -> olm;
+    platform -> config;
+
+    config -> i18n [color=blue];
+    config -> theme [color=blue];
+    config -> skin [color=blue];
+
+    i18n -> modernizr [color=blue];
+    theme -> modernizr [color=blue];
+    skin -> modernizr [color=blue];
+
+    modernizr -> ready [label="Yes"];
+    modernizr -> incompatible [label="No"];
+    incompatible -> ready [label="user ignore"];
+
+    olm -> ready [color=red];
+    config -> ready [color=red];
+    skin -> ready [color=red];
+    theme -> ready [color=red];
+    i18n -> ready [color=red];
+  }
+
+  subgraph cluster_1 {
+    color = green;
+    node [style=filled];
+    label = "init.tsx";
+
+    ready -> loadApp;
+    loadApp -> matrixchat;
+  }
+}
+</code></pre>
+</p>
+</details>
+
+Key:
++ Parallelogram: async/await task
++ Box: sync task
++ Diamond: conditional branch
++ Egg: user interaction
++ Blue arrow: async task is allowed to settle but allowed to fail
++ Red arrow: async task success is asserted
+
+Notes:
++ A task begins when all its dependencies (arrows going into it) are fulfilled.
++ The success of setting up rageshake is never asserted, riot-web has a fallback path for running without IDB (and thus rageshake).
++ Everything is awaited to be settled before the Modernizr check, to allow it to make use of things like i18n if they are successful.
+
+Underlying dependencies:
+![image](https://user-images.githubusercontent.com/2403652/73848977-08624500-4821-11ea-9830-bb0317c41086.png)