[docs] add How it works file

docs/HowItWorks.md

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+# How Does Rector Work?
+
+(Inspired by [*How it works* in BetterReflection](https://github.com/Roave/BetterReflection/blob/master/docs/how-it-works.md))
+
+## 1. Finds all files and Load Configured Rectors
+
+- The application finds files in source you provide and registeres Rectors - from `--level`, `--configur` or local `rector.yml`
+- Then it iterates all found files and applies relevant Rectors to them.
+- *Rector* in this context is 1 single class that modifies 1 thing, e.g. changes class name
+
+## 2. Parse and Reconstruct 1 File
+
+### 2.1 Prepare Phase
+
+- File is parsed by [`nikic/php-parser`](https://github.com/nikic/PHP-Parser), 4.0-dex (this is important, because this version support writing modified tree back to file)
+- Then nodes (array of objects by parser) are traversed by `StandaloneTraverseNodeTraverser` to prepare it's metadata, e.g. class name, method node the node is in, namespace name etc. added by `$node->setAttribute(Attribute::CLASS_NODE, 'value')`.
+
+### 2.2 Rectify Phase  
+ 
+- When all nodes are ready, applicies iterates all active Rector
+- Each nodes is passed to `$rector->isCandidate($node)` method to see, if this Rector should do some work on it, e.g. is this class name called `OldClassName`?
+- If it doesn't match, it goes to next node.
+- If it matches, the `$rector->reconstruct($node)` method is called
+- Active Rector changes all what he should and returns changed node
+
+### 2.3 Save File/Diff Phase
+
+- When work on all nodes of 1 file is done, the file will be saved if it has some changes 
+- Or if the `--dry-run` option is on, it will store the *git-like* diff thanks to [GeckoPackages/GeckoDiffOutputBuilder](https://github.com/GeckoPackages/GeckoDiffOutputBuilder)
+- Then go to next file
+
+## 3 Reporting
+
+- After this Rector displays list of changed files 
+- Or with `--dry-run` option the diff of these files