Add information about daal4py in install.md file for sklearnex (#941)

Author: Pahandrovich

INSTALL.md

@@ -139,7 +139,10 @@ We recommend this installation for the users of Intel® Distribution for Python.
 Intel(R) Extension for Scikit-learn is easily built from sources with the majority of the necessary prerequisites available on conda or pip. The instructions below detail how to gather the prerequisites and build and install the completed package. The package can be built for all three major platforms (Windows, Linux, macOS).
 
 ### Prerequisites
-* Python version >= 3.6
+* Python version >= 3.6, < 3.10
+* daal4py >= 2021.4
+
+**NOTE:** You can [build daal4py from sources](https://github.com/intel/scikit-learn-intelex/blob/master/daal4py/INSTALL.md) or get it from [distribution channels](https://intelpython.github.io/daal4py/#getting-daal4py).
 
 ### Configure the build with environment variables
 * SKLEARNEX_VERSION: sets package version

doc/sources/installation.rst

@@ -133,6 +133,11 @@ Sources are available in |intelex_repo|_.
 ::
 
     Python version >= 3.6, < 3.10
+    daal4py >= 2021.4
+
+.. note::
+    You can `build daal4py from sources <https://github.com/intel/scikit-learn-intelex/blob/master/daal4py/INSTALL.md>`_ or get it from `distribution channels
+    <https://intelpython.github.io/daal4py/#getting-daal4py>`_.
 
 Configure the build with environment variables
 ==============================================