[ci skip] [Zend] Update README.md with new Zend VM executors

Closes GH-8008

As a new learner to PHP Zend VM, the first place I can find information
is this README.md. Unfortunately, it has not included the latest Zend VM
kind and code generation script.

With code reading and trail-and-error experiments, I found a few updates
that might be helpful to new learners.

ChangeLog:
1) Add HYBRID threading model
2) Remove --without-old-executor option and description
3) ZE2 --> ZE3 (Since PHP 7)
4) Change default VM kind from CALL to HYBRID

Signed-off-by: Su, Tao <tao.su@intel.com>

Author: Su, Tao

Zend/README.md

@@ -119,21 +119,21 @@ ZEND_VM_HELPER_EX(<HELPER-NAME>, <OP1_TYPES>, <OP2_TYPES>, <PARAM_SPEC>)
 }
 ```
 
-Executor's code is generated by PHP script zend_vm_gen.php it uses
+The executors code is generated by the PHP script `zend_vm_gen.php`. It uses
 `zend_vm_def.h` and `zend_vm_execute.skl` as input and produces
 `zend_vm_opcodes.h` and `zend_vm_execute.h`. The first file is a list of opcode
 definitions. It is included from `zend_compile.h`. The second one is an executor
 code itself. It is included from `zend_execute.c`.
 
-`zend_vm_gen.php` can produce different kind of executors. You can select
-different opcode threading model using `--with-vm-kind=CALL|SWITCH|GOTO`. You
-can disable opcode specialization using `--without-specializer`. You can include
-or exclude old executor together with specialized one using
-`--without-old-executor`. At last you can debug executor using original
-`zend_vm_def.h` or generated file `zend_vm_execute.h`. Debugging with original
-file requires `--with-lines` option. By default ZE2 uses the following command
-to generate executor:
+`zend_vm_gen.php` can produce different kind of executors. You can select a
+different opcode threading model using `--with-vm-kind=CALL|SWITCH|GOTO|HYBRID`.
+You can disable opcode specialization using `--without-specializer`.
+At last you can debug the executor using the original `zend_vm_def.h` or the
+generated `zend_vm_execute.h` file. Debugging with the original file requires
+the `--with-lines` option. By default ZE3 (since PHP 7) uses the following
+command to generate the executor:
 
 ```bash
-php zend_vm_gen.php --with-vm-kind=CALL
+# Default VM kind is HYBRID
+php zend_vm_gen.php --with-vm-kind=HYBRID
 ```