stm: update README.md for master branch

Signed-off-by: Denis Ciocca <denis.ciocca@st.com>
Change-Id: Ib43cd92cd380dc8ee396ddb07078738fd9a86653

README.md

@@ -1,2 +1,70 @@
-# kernel 5.10.y
-[documentation](https://github.com/STMicroelectronics/st-mems-android-linux-drivers-iio/tree/documentation)
+# Introduction
+This repository contains STMicroelectronics MEMS IIO drivers for Linux/Android kernels.
+
+## Source code integration
+From your kernel source code directory add the git remote (i.e. stmems_iio_github) for this repository:
+```bash
+git remote add stmems_iio_github \
+               https://github.com/STMicroelectronics/st-mems-android-linux-drivers-iio.git
+```
+
+Fetch the just added remote:
+```bash
+git fetch stmems_iio_github
+```
+
+There are now two ways to integrate the drivers code into the kernel target branch:
+* merge (**suggested strategy**)
+* rebase
+
+### merge
+Merge the stmems_iio_github/master with your target kernel source branch (i.e branch linux-5.4.y):
+
+```bash
+git merge --no-fork-point \
+          linux-5.4.y \
+          stmems_iio_github/master
+```
+
+### rebase
+Rebase the stmems_iio_github/master on top of your target kernel source branch (i.e branch linux-5.4.y):
+
+```bash
+git rebase -Xno-renames \
+           --no-fork-point \
+           linux-5.4.y \
+           stmems_iio_github/master
+```
+
+## Apply patches
+Now that drivers code has been added to the target kernel branch, few patches needs to be added in order to:
+* add STM drivers into Kconfig & Makefile systems
+* patch IIO framework with custom events with custom events, channels and devices
+
+Apply the patches available in the just added repository (i.e branch linux-5.4.y):
+
+```bash
+git am stm_iio_patches/5.4.y/*-stm-*.patch
+```
+
+## Configuration
+A folder named stm_iio_configs should now be available with the default configs for the supported drivers.
+
+### Modify target defconfig
+Sensors defconfig can be appended to the board defconfig (i.e. if your current configuration file is arch/arm/configs/stm32_defconfig):
+
+```bash
+cat stm_iio_configs/lsm6dsm_defconfig >> arch/arm/configs/stm32_defconfig
+```
+
+Alternatively, it can be done at build time without altering the board config file, as follow.
+
+### Merge configuration
+Driver config can be merged into current target pre-configured kernel using a script available in the kernel itself:
+
+```bash
+export ARCH=arm
+export CROSS_COMPILE=arm-linux-gnu-
+scrips/kconfig/merge_config.sh -n .config stm_iio_configs/lsm6dsm_defconfig
+```
+