Merge reactive scope refactor

docs/tut/crash-course/5-effect.md

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+# Effect
+
+An effect is a function that is run anytime a source updates. They are called
+effects because they can produce side-effects when reacting to source changes.
+
+Effects are created using `effect()`.
+
+```lua
+local vide = require(vide)
+local source = vide.source
+local effect = vide.effect
+
+local function Counter()
+    local count = source(0)
+
+    effect(function()
+        print("count has updated to: " .. count())
+    end)
+
+    return create "TextButton" {
+        Position = UDim2.fromOffset(300, 300),
+        Size = UDim2.fromOffset(200, 50),
+
+        Text = count,
+
+        Activated = function()
+            count(count() + 1)
+        end
+    }
+end
+
+mount(function() return create "ScreenGui" { Counter {} } end, game.StarterGui)
+```
+
+This will print to the terminal anytime the count is changed.
+
+`effect()` creates an explicit side-effect. There are other side-effects in the
+above code sample. The setting of `Text = count` creates another side-effect;
+the updating of the Text property anytime the count is changed.
+
+All observable changes to the user are considered to be side-effects of the
+reactive system.
+
+You should not update other sources using an effect. Improper usage can lead to
+unecessary updates and infinite loops.