Include information about the benchmark output/methodology

Addresses #930

programs/zstd.1.md

@@ -258,6 +258,9 @@ BENCHMARK
 * `--priority=rt`:
     set process priority to real-time
 
+**Output Format:** CompressionLevel#Filename : IntputSize -> OutputSize (CompressionRatio), CompressionSpeed, DecompressionSpeed
+
+**Methodology:** For both compression and decompression speed, the entire input is compressed/decompressed in-memory to measure speed. A run lasts at least 1 sec, so when files are small, they are compressed/decompressed several times per run, in order to improve measurement accuracy.
 
 ADVANCED COMPRESSION OPTIONS
 ----------------------------