Not compatible to Chef 13+

With chefdk 2.6, chef fails to compile this cookbook:

```
================================================================================
Recipe Compile Error in /tmp/kitchen/cache/cookbooks/ceph-chef/providers/README.md
================================================================================

SyntaxError
-----------
/tmp/kitchen/cache/cookbooks/ceph-chef/providers/README.md:3: syntax error, unexpected tIDENTIFIER, expecting keyword_do or '{' or '('
k. It allows for easy creation, setting and deleting of pool
^
/tmp/kitchen/cache/cookbooks/ceph-chef/providers/README.md:3: syntax error, unexpected keyword_and, expecting '='
for easy creation, setting and deleting of pools. It allows
^
/tmp/kitchen/cache/cookbooks/ceph-chef/providers/README.md:3: syntax error, unexpected tIDENTIFIER, expecting keyword_do or '{' or '('
It allows for setting replica size and types such as replic
^
/tmp/kitchen/cache/cookbooks/ceph-chef/providers/README.md:5: syntax error, unexpected tINTEGER, expecting '('
2. client.rb
^
/tmp/kitchen/cache/cookbooks/ceph-chef/providers/README.md:6: syntax error, unexpected tIDENTIFIER, expecting keyword_do or '{' or '('
der allows for simple creation of basic ceph clients such as
^
/tmp/kitchen/cache/cookbooks/ceph-chef/providers/README.md:6: unterminated string meets end of file

System Info:
------------
chef_version=13.11.3
platform=ubuntu
platform_version=16.04
ruby=ruby 2.4.4p296 (2018-03-28 revision 63013) [x86_64-linux]
program_name=chef-client worker: ppid=12482;start=08:58:04;
executable=/opt/chef/bin/chef-client
```

Apparently, it tries to parse the README.md as ruby file…