readme: Add readme

Add README.md that identifies the origin of Segger source files and
provide a procedure for migrating Zephyr changes to new Segger releases.

Signed-off-by: Bryce Wilkins <[email protected]>

Author: brycewilkins

README.md

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+## Segger module for Zephyr RTOS
+
+This repository is the Zephyr project module for integrating Segger [RTT](https://www.segger.com/products/debug-probes/j-link/technology/about-real-time-transfer) ([wiki](https://wiki.segger.com/RTT)), [SystemView](https://www.segger.com/products/development-tools/systemview/) ([wiki](https://wiki.segger.com/SystemView)) and [Monitor mode debugging](https://www.segger.com/products/debug-probes/j-link/technology/monitor-mode-debugging/) with Zephyr RTOS.
+
+The code here is predominantly untouched public Segger source code with changes to support Zephyr RTOS.
+
+This README identifies the origin of upstream Segger files and suggests a procedure to follow when updating to a new Segger release.
+
+### Upstream Segger source code
+Segger's [official GitHub repository](https://github.com/SEGGERMicro) is not currently actively maintained, hence Segger code is obtained from publicly accessible download links posted on Segger's website.
+
+#### RTT and SystemView
+[Download](https://www.segger.com/downloads/jlink/systemview_target_src) "SystemView, Target Sources" from the SystemView downloads [page](https://www.segger.com/downloads/systemview/). This download contains both RTT and SystemView sources.
+
+#### Monitor mode debugging
+[Download](https://www.segger.com/downloads/pub/Generic_Cortex-M_MonitorModeSystickExample_SES.zip) "Example project" from the [Monitor mode debugging](https://www.segger.com/products/debug-probes/j-link/technology/monitor-mode-debugging/) webpage.
+
+### Updating to a new Segger release
+Upgrading to a new Segger release means migrating existing Zephyr changes to the latest Segger code. There are many tools (`diff`, `patch`, `git diff`, etc.) and different approaches to accomplishing this -- use what you are familiar with. The following procedure is a guide to achieving the desired result.
+
+1. Obtain the updated Segger code and prepare for subsequent steps
+    1. Download upstream code from the links above
+    2. Extract the SystemView update and delete the `Samples` directory
+    3. Extract the Monitor mode example and delete _non_-JLINK_MONITOR* files
+    4. Organize the files to match the directory structure of this repository
+    5. Convert line endings of all files to Unix style LF (`\n`)
+    6. Strip trailing white space of all files
+2. Establish the Zephyr patches that will be applied in step 3
+    1. Download the SystemView release this repository is _currently_ based on (refer to "SystemView version: x.yy" in the source files to determine the current release)
+    2. Repeat step 1 parts ii-vi for these downloaded files
+    3. Compare the files of this repository to those of step 2 part ii to identify the existing Zephyr changes (the "diff", or "patch") needed for step 3
+3. Apply the Zephyr changes to the new Segger release
+    1. Apply the patches from step 2 part iii to the files of step 1
+    2. Sanity check the result and revise as needed
+4. Review/update the list of patched files below
+5. Commit the updated + modified Segger files and push the change
+6. Open a pull-request to merge the change into this repository
+
+### Patched files
+Segger source files with Zephyr specific changes:
+1. `Config/SEGGER_RTT_Conf.h`
+    * Defines related to RTT buffers, memory region, locking mechanism
+2. `Config/SEGGER_SYSVIEW_Conf.h`
+    * Defines related to SystemView buffers, memory region, locking mechanism
+3. `SEGGER/SEGGER_RTT.c`
+    * RTT initialization options