Add python virtualenv to gitignore.

.gitignore

@@ -24,3 +24,6 @@
 
 # Makefile
 /Makefile
+
+# Python virtualenv
+/venv

docs/howto/how-to-cross-build-runtime-for-arm.md

@@ -104,17 +104,55 @@ ACL source will be automatically installed in `externals/ARMCOMPUTE` when you bu
 
 You can check ACL source information in `infra/cmake/packages/ARMComputeSourceConfig.cmake`
 
+## Install numpy
+
+Python package `numpy` is needed for build. It's recommended to always install python packages in virtualenv.
+To do this, at first you need to install python venv package:
+
+```
+$ sudo apt-get install python3-venv
+```
+Next, create virtualenv:
+
+```
+$ python3 -m venv venv
+```
+
+and activate it (please keep in mind that you will have to activate it always before build)
+
+```
+$ source /venv/bin/activate
+```
+
+Next, just in case upgrade pip package:
+
+```
+(venv)$ pip install -U pip
+```
+
+and finally install `numpy`:
+
+```
+(venv)$ pip install numpy
+```
+
 ## Cross build for ARM by using Makefile.template
 
 Give `TARGET_ARCH` variable to set the target architecture.
 
 If you used `ROOTFS_DIR` to prepare in alternative folder, you should also give this to makefile.
 
+Remember to activate venv if you didn't activate it already:
+
+```
+$ source /venv/bin/activate
+```
+
 ```
-$ CROSS_BUILD=1 TARGET_ARCH=armv7l make -f Makefile.template
+(venv)$ CROSS_BUILD=1 TARGET_ARCH=armv7l make -f Makefile.template
 
 # If ROOTFS_DIR is in alternative folder
-$ ROOTFS_DIR=/path/to/your/rootfs/arm \
+(venv)$ ROOTFS_DIR=/path/to/your/rootfs/arm \
 CROSS_BUILD=1 TARGET_ARCH=armv7l make
 ```