Add build bloat readme

ydb/ci/build_bloat/README.md

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+# Build bloat
+
+A tool used to visualize clang's compilation profiles.
+
+![screenshot](screenshot.png)
+
+It helps to answer the following questions:
+- which dirs/files has longest impact on build time
+- which headers has longest impact on build time
+
+## How to use
+
+You have to generate compilation profiles using `-ftime-trace` as compilation flag. 
+
+With `ya` tool it can be done with folowing comand:
+
+`ya make <your usual arguments> --output=~/some_output_build_dir -DCOMPILER_TIME_TRACE --add-result=.json`
+
+After build is done run:
+
+`ya tool python3 main.py --build-dir ~/some_output_build_dir --html-dir-cpp html_cpp_impact --html-dir-headers html_headers_impact`
+
+And open `html_cpp_impact/index.html`, `html_headers_impact/index.html`