scan code is not checking .txt files for license

In the openwhisk-kube-deploy project, the file home/templates/Notes.txt does not have a license header. However, scancode with the ASF-Release.cfg is not detecting this because it appears to not be scanning any file with a .txt extension for a license.

Appended is the relevant snippet from the verbose output of the tool.

```
[/Users/dgrove/code/openwhisk/incubator-openwhisk-deploy-kube/tools/travis/../../NOTICE.txt]...
 Running Line Check [regex_check]
[/Users/dgrove/code/openwhisk/incubator-openwhisk-deploy-kube/tools/travis/../../LICENSE.txt]...
 Running Line Check [regex_check]
[/Users/dgrove/code/openwhisk/incubator-openwhisk-deploy-kube/tools/travis/../../helm/templates/NOTES.txt]...
 Running Line Check [regex_check]
[/Users/dgrove/code/openwhisk/incubator-openwhisk-deploy-kube/tools/travis/../../.travis.yml]...
 Running File Check [has_block_license]
 Running Line Check [no_trailing_spaces]
 Running Line Check [eol_at_eof]
 Running Line Check [regex_check]

```

Start with the ASF-Release.cfg behavior shown in the verbose scancode output, comparing the checks run for NOTICE.txt, LICENSE.txt, NOTES.txt, and .travis.yml. Trace how file extensions are selected and verify that home/templates/Notes.txt is checked for a license header, with the scan reporting the missing header.

Written by the indexing model from the issue text.