Snapcraft complain on a non-executable hook named `README.md`

[林博仁(Buo-ren, Lin)]

Error message:

```
Determining the version from the project repo (version-script).
The version has been set to '2.8.1-d+pkg-8c01'
hook 'README.md' is not executable
Build failed
```

This causes trouble on documenting the sub-dirs under /snap[1]. Since according to [Supported snap hooks - doc - snapcraft.io](https://forum.snapcraft.io/t/supported-snap-hooks/3795) snapcraft requires hooks to be named to certain keywords it shouldn't care about files that aren't named that way.

[1] https://github.com/Lin-Buo-Ren/snapcrafters-template-plus/tree/master/snap/hooks

[Michał Sawicz (saviq)]

Somewhat related: #1812003

[John Lenton (chipaca)]

The error is probably from 'snap check', which snapcraft calls to perform static analysis.
snapd is picky about what you put in the hooks directory, and this is unlikely to change. Having said that, we could have it ignore non-executable README* files. We should discuss this.