Added 'Install' section to systemd example in README

[marcomandola]

What type of PR is this?

/kind documentation

Any specific area of the project related to this PR?

/area config

What this PR does / why we need it:

This PR adds the [Install] section in the Systemd Installation example in README.
This is required for enabling and disabling services, as per RedHat Documentation.

When following the systemd installation example, sudo systemctl enable falcosidekick currently fails with the following error:

The unit files have no installation config (WantedBy=, RequiredBy=, Also=,
Alias= settings in the [Install] section, and DefaultInstance= for template
units). This means they are not meant to be enabled using systemctl.

Possible reasons for having this kind of units are:
• A unit may be statically enabled by being symlinked from another unit's
  .wants/ or .requires/ directory.
• A unit's purpose may be to act as a helper for some other unit which has
  a requirement dependency on it.
• A unit may be started when needed via activation (socket, path, timer,
  D-Bus, udev, scripted systemctl call, ...).
• In case of template units, the unit is meant to be enabled with some
  instance name specified.

Which issue(s) this PR fixes:

N/A

Special notes for your reviewer:

[poiana]

Welcome @marcomandola! It looks like this is your first PR to falcosecurity/falcosidekick 🎉

[Issif]

Thanks

[poiana]

LGTM label has been added.

Git tree hash: 7503c81bd64330aebef5af0bd170905b5707ba8d

[poiana]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Issif, marcomandola

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