Build helpers: Create README.md

Author: howetuft

build-helpers/README.md

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+## Helpers and other stuff for Build process
+
+Root contents:
+- `build-settings.json`: static settings for build process, including version of the dependencies. Used by CMake, Github Workflows and others.
+- `ccache.conf`: configuration for ccache. Used by Github Workflows
+
+Folder contents:
+- `./cmake`: additional modules for CMake. Used by CMakeLists.txt.
+- `./conan`: project's conan recipe. Used by `make` helpers (see below)
+- `./debug`: (handcrafted) scripts for build debugging. Run on command line
+- `./make`: underlying Python scripts of `make` wrapper - enabling `make deps`, `make <target>`, `make clean` etc. Called by `GNUMakefile` and `make.bat`
+- `./wheels`: helpers for wheel building. Called by Github Workflows.