add a section on restricted support of environment variables to README.md

diff --git a/programs/README.md b/programs/README.md
index ca9056eaaa43711280414555148ac7beb27a3f32..baf90e88fffa7a9ea359f18caaf4e1449cc76abf 100644 (file)
--- a/programs/README.md
+++ b/programs/README.md
@@ -172,6 +172,10 @@ Benchmark arguments :
 --priority=rt : set process priority to real-time
 ```
 
+#### Restricted usage of Environment Variables
+Using environment variables to set compression/decompression parameters has security implications. Therefore,
+we intentionally restrict its usage. Currently, only `ZSTD_CLEVEL` is supported for setting compression level.
+If the value of `ZSTD_CLEVEL` is not a valid integer, it will be ignored with a warning message.
 
 #### Long distance matching mode
 The long distance matching mode, enabled with `--long`, is designed to improve