README.md in pycall-1.2.1 vs README.md in pycall-1.3.0.dev

@@ -59,9 +59,48 @@
     math.sin(math.pi / 4) - Math.sin(Math::PI / 4)   # => 0.0
 
 Type conversions from Ruby to Python are automatically performed for numeric,
 boolean, string, arrays, and hashes.
 
+### Releasing the RubyVM GVL during Python function calls
+
+You may want to release the RubyVM GVL when you call a Python function that takes very long runtime.
+PyCall provides `PyCall.without_gvl` method for such purpose.  When PyCall performs python function call,
+PyCall checks the current context, and then it releases the RubyVM GVL when the current context is in a `PyCall.without_gvl`'s block.
+
+```ruby
+PyCall.without_gvl do
+  # In this block, all Python function calls are performed without
+  # the GVL acquisition.
+  pyobj.long_running_function()
+end
+
+# Outside of PyCall.without_gvl block,
+# all Python function calls are performed with the GVL acquisition.
+pyobj.long_running_function()
+```
+
+### Debugging python finder
+
+When you encounter `PyCall::PythonNotFound` error, you can investigate PyCall's python finder by setting `PYCALL_DEBUG_FIND_LIBPYTHON` environment variable to `1`.  You can see the log like below:
+
+```
+$ PYCALL_DEBUG_FIND_LIBPYTHON=1 ruby -rpycall -ePyCall.builtins
+DEBUG(find_libpython) find_libpython(nil)
+DEBUG(find_libpython) investigate_python_config("python3")
+DEBUG(find_libpython) libs: ["Python.framework/Versions/3.7/Python", "Python", "libpython3.7m", "libpython3.7", "libpython"]
+DEBUG(find_libpython) libpaths: ["/opt/brew/opt/python/Frameworks/Python.framework/Versions/3.7/lib", "/opt/brew/opt/python/lib", "/opt/brew/opt/python/Frameworks", "/opt/brew/Cellar/python/3.7.2_1/Frameworks/Python.framework/Versions/3.7", "/opt/brew/Cellar/python/3.7.2_1/Frameworks/Python.framework/Versions/3.7/lib"]
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/Frameworks/Python.framework/Versions/3.7/lib/Python.framework/Versions/3.7/Python
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/Frameworks/Python.framework/Versions/3.7/lib/Python.framework/Versions/3.7/Python.dylib
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/Frameworks/Python.framework/Versions/3.7/lib/darwin/Python.framework/Versions/3.7/Python
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/Frameworks/Python.framework/Versions/3.7/lib/darwin/Python.framework/Versions/3.7/Python.dylib
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/lib/Python.framework/Versions/3.7/Python
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/lib/Python.framework/Versions/3.7/Python.dylib
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/lib/darwin/Python.framework/Versions/3.7/Python
+DEBUG(find_libpython) Unable to find /opt/brew/opt/python/lib/darwin/Python.framework/Versions/3.7/Python.dylib
+DEBUG(find_libpython) dlopen("/opt/brew/opt/python/Frameworks/Python.framework/Versions/3.7/Python") = #<Fiddle::Handle:0x00007fc012048650>
+```
+
 ## PyCall object system
 
 PyCall wraps pointers of Python objects in `PyCall::PyPtr` objects.
 `PyCall::PyPtr` class has two subclasses, `PyCall::PyTypePtr` and
 `PyCall::PyRubyPtr`.  `PyCall::PyTypePtr` is specialized for type (and classobj