Update README.md

README.md

diff --git a/README.md b/README.md
index 2a8f427..aea768a 100644
--- a/README.md
+++ b/README.md
@@ -254,6 +254,7 @@
 basisu -test_hdr_6x6
 basisu -test_hdr_6x6i
 basisu -test_xuastc_ldr
+basisu -test_codecs
 ```
 
 To test the codec in OpenCL mode (must have OpenCL libs/headers/drivers installed and have compiled OpenCL support in by running cmake with `-DBASISU_OPENCL=TRUE`):
@@ -262,6 +263,8 @@
 basisu -test -opencl
 ```
 
+A new test mode, enabled via `-test_codecs`, was added in v2.5. This option tests all the codecs in various ways.
+
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 Compressing and Unpacking .KTX2/.basis Files