Add info on version 1.5.3

Author: BlueM

README.markdown

@@ -9,7 +9,7 @@ What is it?
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 `Tree` and `Tree\Node` are PHP classes for handling data that is structured hierarchically using parent ID references. A typical example is a table in a relational database where each record’s “parent” field references the primary key of another record. Of course, Tree cannot only use data originating from a database, but anything: you supply the data, and Tree uses it, regardless of where the data came from and how it was processed.
 
-It is important to know that the tree structure created by this package is read-only: you can’t use it to perform modifications (add, update, delete, reorder) of the tree nodes.
+It is important to know that the tree structure created by this package is *read-only*: you can’t use it to perform modifications of the tree nodes. If you need a library for that, you might want to take a look at [nicmart/tree](https://github.com/nicmart/Tree).
 
 On the other hand, one nice thing is that it’s pretty fast. This does not only mean the code itself, but also that the constructor takes the input data in a format that is simple to create. For instance, to create a tree from database content, a single `SELECT` is sufficient, regardless of the depth of the tree and even for thousands of nodes.
 
@@ -142,6 +142,10 @@ To be able to use a custom node class you must extend `BlueM\Tree` and overwrite
 Version History
 =================
 
+1.5.3
+-----
+* Handle IDs of mixed type (strings and integers)
+
 1.5.2
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 * Add info on JSON serialization in Readme. No code changes.