little bit of docs

README.md

@@ -8,6 +8,33 @@ If you're looking  to use the classes generated by this library, you may want th
 
 Please refer to our wiki for the latest documentation: https://github.com/dcarbone/php-fhir/wiki
 
+# V4 Changes
+
+Version 4 of this libary constitutes a near-complete rewrite of the builder and resulting code.  The primary goals
+of this rewrite were to:
+
+1. Make constructing types easier / less tedious:
+   * Type constructors now have a parameter per field, rather than accepting an nested assocative array of values.
+   * Promoting use of PHP 8.0+ named parameters (https://www.php.net/manual/en/functions.arguments.php#functions.named-arguments).
+2. Improve performance of generated code.
+   * Type properties no longer carry a "default" value of null or empty array.  They are now either set to a non-empty
+     value or are not set at all.
+   * JSON serialization in particular no longer uses `$out->{self::FIELD_ID} = $v;` syntax for setting fields, favoring setting
+     the field directly via `$out->id = $v;`.  In general, more direct use of properties is made.
+3. Separate core classes, interfaces, traits, enums, etc., from FHIR resource classes.
+   * "Core" classes are now present under the root namespace, with each generated version in a separate sub-namespace.
+   * "Version Type" classes are further sub-namespaced under each version namespace to separate them from "Version Core" classes.
+4. Provide a basic API client
+   * This is still very much a work in progress, but the goal is to provide a simple, easy-to-use API client for FHIR servers.
+5. Separate "Core" entities into their own namespace
+   * "Core" entities are now separated into common-purpose namespaces, such as `Encoding`, `Client`, `Validation`, etc.
+6. Improve and simplify runtime configuration
+   * I have created two core config classes:
+       * `Encoding\UnserializeConfig` - Configures both JSON and XML unserialization options
+       * `Encoding\SerializeConfig` - Configures both JSON and XML serialization options
+7. Simplify templates
+   * This is still a work in progress, and will probably drastically change again.
+
 ## Parsing Example
 
 ```php