Directory Structure Spec

[shizhMSFT]

Resolves #167

Signed-off-by: Shiwei Zhang shizh@microsoft.com

[SteveLasker]

I've added Balancing Security and Usability for reference

Based on the PR, it looks like we are securing the configuration and policy files to directories a normal user would be unable to configure. We should also assure the notation client doesn't allow the user to override the configuration files with non-admin directories or files.
There may be some configurations that are less secure, and more usability, for instance reducing the number of parameters the user must enter every time. However, anything that implies security, MUST be placed in a directory requiring admin rights for write access.

[gokarnm]

This makes sense from a usability perspective for certain components like configuration settings, signing key and plugin config, and local keys, but seems inverse of what is desired for trust store and trust policy.

[gokarnm]

Suggested change

	On Unix systems, `notation` follows [Filesystem Hierarchy Standard][FHS] for system level directories and [XDG Base Directory Specification][XDG] for user level directories. On Windows, [Known Folders][KF] and [App Settings][AS] are followed equivalently. On Darwin, [macOS File System][macOS_FS] with [System Integrity Protection][SIP] is followed equivalently. If a file with the same name exists at the system level and the user level, the file at the user level takes over the priority.
	On Unix systems, `notation` follows [Filesystem Hierarchy Standard][FHS] for system level directories and [XDG Base Directory Specification][XDG] for user level directories. On Windows, [Known Folders][KF] and [App Settings][AS] are followed equivalently. On Darwin, [macOS File System][macOS_FS] with [System Integrity Protection][SIP] is followed equivalently.
	**TODO** : If a file with the same name exists at the system level and the user level, the mechanism to resolve priority needs to be defined (tracked in #203).

[gokarnm]

Created an issue (#203) and added a suggestion to track precedence of system vs user files. Rest looks great!