Revise README.md to clarify multithreading details

Updated documentation on the use and impacts of multithreading in the context of parallel validation of CSV by columns. Additionally, explained the potential downside of allocating more threads than available CPU cores on performance due to system overhead.

README.md

@@ -1429,6 +1429,7 @@ effectively.
   available CPU cores.
 - **Thread Pool Size:** You can set a specific number of threads for the pool. For example, `--parallel=10` will set the
   thread pool size to 10. It doesn't make much sense to specify more than the number of logical cores in your CPU.
+  Otherwise, it will only slow things down a bit due to the system overhead to handle multithreading.
 - **Disabling Parallelism:** Using `--parallel=1` disables parallel processing, which is the default setting if the
   option is not specified.
 - **Implementation:** The feature relies on the `ext-parallel` PHP extension, which enables the creation of lightweight
@@ -1935,8 +1936,7 @@ It's random ideas and plans. No promises and deadlines. Feel free to [help me!](
 
 * **Performance and optimization**
     * Using [vectors](https://www.php.net/manual/en/class.ds-vector.php) instead of arrays to optimaze memory usage and speed of access.
-    * Parallel validation of schema by columns. You won't believe this, but modern PHP has multithreading support.
-    * Parallel validation of multiple files at once.
+    * Multithreading support for parallel validation of CSV by columns.
 
 * **Mock data generation**
     * Create CSV files based on the schema (like "create 1000 rows with random data based on schema and rules").