Adding docs for always decompile blocks (#360)

* Adding docs for always decompile blocks

* Renaming file and adding example

* Adding link to javascript blocks

docs/blocks.md

@@ -3,7 +3,7 @@
 ### @description Language constructs for the Block editor.
 
 Blocks snap into each other to define the program that your @boardname@ will run.
-Blocks can be event (buttons, shake, ...) or need to be snapped into an event to run. 
+Blocks can be event (buttons, shake, ...) or need to be snapped into an event to run.
 The [on-start](/blocks/on-start) event runs first.
 
 ```namespaces
@@ -15,4 +15,4 @@ Math.random(5);
 
 ## See Also
 
-[logic](/blocks/logic), [loops](/blocks/loops), [math](/blocks/math), [variables](/blocks/variables), [on-start](/blocks/on-start)
+[logic](/blocks/logic), [loops](/blocks/loops), [math](/blocks/math), [variables](/blocks/variables), [on-start](/blocks/on-start), [javascript blocks](/blocks/javascript-blocks)

docs/blocks/javascript-blocks.md

@@ -0,0 +1,65 @@
+# JavaScript Block
+
+JavaScript is a very powerful language; so powerful that some concepts in
+JavaScript can't be shown using blocks. When PXT encounters a piece of code
+that can't be converted into blocks, it instead creates a grey JavaScript block
+to preserve it. These blocks get converted right back into the original
+JavaScript when you switch back to the text editor.
+
+```blocks
+for (let index = 0; index <= 5; index++) {
+    basic.showNumber(fibonacci(index))
+}
+
+function fibonacci(n: number): number {
+    if (n === 0 || n === 1) {
+        return 1;
+    }
+
+    return fibonacci(n - 1) + fibonacci(n - 2);
+}
+```
+
+The code in JavaScript blocks cannot be edited, but the blocks
+themselves can be moved, copied, pasted, and deleted just like any
+other block.
+
+
+## How do I get my JavaScript code to convert without any grey blocks?
+
+The easiest way to write code that will convert cleanly back to blocks
+is to write the code using blocks and convert it to JavaScript. Many blocks
+have a very specifc structure to their JavaScript that must be
+followed in order to have them convert properly. If you have a
+piece of your code that does not convert, try looking at the block
+you were expecting and the JavaScript it produces.
+
+For example, a for-loop must have this structure to convert to a block:
+
+```typescript
+for (let x = 0; x <= 5; x++) {
+}
+```
+
+The following examples can't be represented by the blocks:
+
+```typescript
+// Condition must be either < or <= with a variable on the left side
+for (let x = 0; x > -1; x++) {
+}
+
+// The variable in the condition and incrementor must be the declared variable
+for (let x = 0; x <= 5; y++) {
+}
+
+// The declared variable must be initialized to 0
+for (let x = 2; x <= 5; x++) {
+}
+
+// All three parts of the for-loop must be present
+for (;;) {
+}
+```
+
+Try to match the JavaScript of the desired block as closely as
+possible.