docs(snapshot): document dm-snapshot kernel module is required.

[flokli]

Why is this PR required?: https://github.com/openebs/lvm-localpv/issues/163

What this PR does?: It documents the fact the dm-snapshot kernel module needs to be loaded

Does this PR require any upgrade changes?: no

If the changes in this PR are manually verified, list down the scenarios covered:: I added a modprobe dm-snapshot; echo dm-snapshot >> /etc/modules to the installation script and verified this works afterwards.

Any additional information for your reviewer? : To fully solve https://github.com/openebs/lvm-localpv/issues/163, there still needs to be some more graceful handling (no crazy busylooping) if the module is not present. Maybe backoff in case the invoked command doesn't succeed)

Checklist:

• [ ] Fixes #
• [ ] PR Title follows the convention of <type>(<scope>): <subject>
• [ ] Has the change log section been updated?
• [ ] Commit has unit tests
• [ ] Commit has integration tests
• [ ] (Optional) Are upgrade changes included in this PR? If not, mention the issue/PR to track:
• [ ] (Optional) If documentation changes are required, which issue on https://github.com/openebs/openebs-docs is used to track them:

[pawanpraka1]

@flokli, would you like to mention your use case in our adopters(openebs/openebs#2719) file.

[zimbatm]

I just noticed that https://github.com/openebs/lvm-localpv#prerequisites is also missing that requirement.

[flokli]

@zimbatm, can you send a PR adding it there, too?

[flokli]

@zimbatm I opened https://github.com/openebs/lvm-localpv/pull/168.