add readme

README.md

@@ -0,0 +1,17 @@
+This is a program to approximately unfold phonon spectra.
+
+Currently, there is no documentation, and some not core features are in other branches
+  and implemented in a temporary way, e.g., directly writing data in source file.
+
+I will finish the documentation and merge these features into main branch ASAP.
+
+The program is packaged in a standard way of cmake, so that you can build it just like other cmake projects:
+
+```bash
+mkdir build
+cd build
+cmake ..
+make
+```
+
+and view source files for input and output format.