update README

README.md

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-# Plsa_parella
-test
+# Plsa_parella code (C++)
+This is an example C++ code for PLSA algorthim and this version is a multi thread version. The code used the boost library so if you wanted compile it and use it correctly you must install the boost lib in your system.
+
+##install & compile
+
+ - step 1: if you have install the boost library you can directly jump to step 2,either donwload the boost library and install it from:[http://www.boost.org/](http://www.boost.org/)
+ - step 2: `./mk.sh`
+
+##use usage
+`./showPlsa_parella 20`
+when the train process is end, the most frequent 20 words of every topic will print out in your screen
+
+