fix: README -> README.md

ta6ob/lz4/contrib/gen_manual/README.md

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+gen_manual - a program for automatic generation of manual from source code
+==========================================================================
+
+#### Introduction
+
+This simple C++ program generates a single-page HTML manual from `lz4.h`.
+
+The format of recognized comment blocks is following:
+- comments of type `/*!` mean: this is a function declaration; switch comments with declarations
+- comments of type `/**` and `/*-` mean: this is a comment; use a `<H2>` header for the first line
+- comments of type `/*=` and `/**=` mean: use a `<H3>` header and show also all functions until first empty line
+- comments of type `/*X` where `X` is different from above-mentioned are ignored
+
+Moreover:
+- `LZ4LIB_API` is removed to improve readability
+- `typedef` are detected and included even if uncommented
+- comments of type `/**<` and `/*!<` are detected and only function declaration is highlighted (bold)
+
+
+#### Usage
+
+The program requires 3 parameters:
+```
+gen_manual [lz4_version] [input_file] [output_html]
+```
+
+To compile program and generate lz4 manual we have used:
+```
+make
+./gen_manual.exe 1.7.3 ../../lib/lz4.h lz4_manual.html
+```