Update README.md

Author: bayesfactor

README.md

@@ -1,2 +1,14 @@
 # mysql-percentile
-user-defined function to calculate percentiles in a MySQL query
+user-defined aggregate function to calculate percentiles in a MySQL query, similar to min/max/avg.
+
+Compile with:
+gcc -fPIC -Wall -I /usr/include/mysql51/mysql/ -shared -o udf_percentile.so  udf_percentile.cc
+
+Copy to the plugin directory of MySQL (SHOW VARIABLES LIKE 'plugin_dir')
+cp udf_percentile.so /usr/lib64/mysql/plugin
+
+Register the function:
+CREATE AGGREGATE FUNCTION percentile RETURNS REAL SONAME 'udf_percentile.so'
+
+Use the function:
+SELECT percentile(loadTime, 99) FROM user_data GROUP BY WEEK(`timestamp`)