Add instrgen how-it-works.md (#3659)

* Add instrgen how-it-works.md

* update image

* fixing point 3

* fixing entry point name

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

* Update instrgen/docs/how-it-works.md

Co-authored-by: Tyler Yahn <[email protected]>

---------

Co-authored-by: Przemek Delewski <[email protected]>
Co-authored-by: Tyler Yahn <[email protected]>

Author: pdelewski

instrgen/README.md

@@ -1,6 +1,7 @@
 # OpenTelemetry Go Source Automatic Instrumentation
 
 This package provides a code generation utility that instruments existing source code with [OpenTelemetry].
+If you are looking for more details about internal working, see [How it works](./docs/how-it-works.md).
 
 ## Project Status
 

instrgen/docs/flow.png

@@ -0,0 +1,12 @@
+## OpenTelemetry Go Source Automatic Instrumentation - How it works
+
+`instrgen` adds OpenTelemetry instrumentation to source code by directly modifying it.
+It uses the AST (Abstract Syntax Tree) representation of the code to determine its operational flow and injects necessary OpenTelemetry functionality into the AST.
+
+The AST modification algorithm is the following:
+1. Search for the entry point: a function definition with `AutotelEntryPoint()`.
+2. Build the call graph. Traverse all calls from the entry point through all function definitions.
+3. Inject OpenTelemetry instrumentation into functions bodies.
+4. Context propagation. Adding an additional context parameter to all function declarations and function call expressions that are visible
+   (it will not add a context argument to call expressions if they are not reachable from the entry point).
+![image info](./flow.png)