Update features with warning for CCDCorrection

doc/features.md

@@ -12,7 +12,7 @@ API : [nabu.preproc](apidoc/nabu.preproc)
 
 This is usually the first step done on the radios. Each radio is normalized by the incoming beam intensity and CCD dark current as `radio = (radio - dark)/(flat - dark)`.
 
-Usually, several "flats" images are acquired. Nabu automatically determines which flat image to use depending on the radio angle.
+Usually, several "flats" images are acquired. Nabu automatically determines which flat image to use depending on the radio angle. A linear interpolation is done between flats to estimate the "current" flat to use.
 
 Configuration file: section `[preproc]`: `flatfield_enabled = 1`.
 
@@ -27,6 +27,9 @@ Configuration file: section `[preproc]`: `ccd_filter_enabled = 1` and `ccd_filte
 
 API: [CCDCorrection](apidoc/nabu.preproc.ccd.rst#nabu.preproc.ccd.CCDCorrection) and [CudaCCDCorrection](apidoc/nabu.preproc.ccd_cuda.rst#nabu.preproc.ccd_cuda.CudaCCDCorrection)
 
+``` warning:: In version 2020.2.0, this feature is not available through the command line interface, meaning that you cannot use it via the configuration file yet. It is only available in the library.
+```
+
 ### Double flat-field
 
 This is a projections-based rings artefacts removal.