HACKING.rst: add clarifying note to LP CLA process section

[OddBloke]

Proposed Commit Message

HACKING.rst: add clarifying note to LP CLA process section

We've had some first time contributors still end up trying to submit via
Launchpad: this commit underlines that this is not necessary.

Test Steps

Build the docs and observe:

Checklist:

• My code follows the process laid out in the documentation
• I have updated or added any unit tests accordingly
• I have updated or added any documentation accordingly

[mitechie]

After going through the process I think that changing the title of this section to set the condition scope would be useful.

e.g.

Transferring CLA Signatures from Launchpad to Github

to perhaps

If you previously signed the CLA through Launchpad...

Which helps set the optional/condition of this section upfront.

The note in this PR is +1 either way.