[doc] Include files from included files in subdirectories needs documenting

[jsseidel]

In the following case, mktechdocs will fail to build because included files in subdirectories must give a path relative to the project directory rather than the included subdirectory.

myproject
    |
    +----> master.md
    +----> subdir1
               |
               + ----> somedoc.md
               |
               + ---- > included.md

In this case, master.md might be:

```include
subdir1/somedoc.md
````

And, you might think somedoc.md should be:

```include
included.md
```

But this won't build correctly. Mktechdocs will skip included.md because it can't find it. Instead, somedoc.md should be:

```include
subdir1/included.md
```

This may or may not be ideal, but it should at least be documented.