docs(subscription): add mention of auto-leasing

[callmehiphop]

Relates to #115

• Tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)

[callmehiphop]

/cc @toots

[codecov]

Codecov Report

Merging #116 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #116   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           8      8           
  Lines         867    867           
=====================================
  Hits          867    867

Impacted Files	Coverage Δ
src/subscription.js	100% <ø> (ø)	⬆️

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[googlebot]

So there's good news and bad news.

👍 The good news is that everyone that needs to sign a CLA (the pull request submitter and all commit authors) have done so. Everything is all good there.

😕 The bad news is that it appears that one or more commits were authored or co-authored by someone other than the pull request submitter. We need to confirm that all authors are ok with their commits being contributed to this project. Please have them confirm that here in the pull request.

Note to project maintainer: This is a terminal state, meaning the cla/google commit status will not change from this state. It's up to you to confirm consent of the commit author(s) and merge this pull request when appropriate.

[toots]

Thanks guys!