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]>

README.md

@@ -0,0 +1,67 @@
+## Segger module for Zephyr RTOS
+
+This repository is the Zephyr project module for integrating Segger [RTT][1] ([wiki][2]),
+[SystemView][3] ([wiki][4]) and [Monitor mode debugging][5] 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][6] is not currently actively maintained, hence Segger code is
+obtained from publicly accessible download links posted on Segger's website.
+
+#### RTT and SystemView
+[Download][7] "SystemView, Target Sources" from the SystemView downloads [webpage][8]. This
+download contains both RTT and SystemView sources.
+
+#### Monitor mode debugging
+[Download][9] "Example project" from the Monitor mode debugging [webpage][10].
+
+### 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
+
+[1]: https://www.segger.com/products/debug-probes/j-link/technology/about-real-time-transfer
+[2]: https://wiki.segger.com/RTT
+[3]: https://www.segger.com/products/development-tools/systemview
+[4]: https://wiki.segger.com/SystemView
+[5]: https://www.segger.com/products/debug-probes/j-link/technology/monitor-mode-debugging
+[6]: https://github.com/SEGGERMicro
+[7]: https://www.segger.com/downloads/jlink/systemview_target_src
+[8]: https://www.segger.com/downloads/systemview
+[9]: https://www.segger.com/downloads/pub/Generic_Cortex-M_MonitorModeSystickExample_SES.zip
+[10]: https://www.segger.com/products/debug-probes/j-link/technology/monitor-mode-debugging