Add notes for non-reflecting boundary condition treatment

[yungyuc]

The third set of NRBC has been implemented with solvcon/modmesh#326 but there are not corresponding notes here in mmnote. More NRBC treatments are planned with solvcon/modmesh#330.

We should add all the NRBC treatment notes here in mmnote.

[yungyuc]

@Gene0315 , as we discussed on discord, let's set a check point to review your NRBC notes added in this repository by 15th June (Sat).

[Gene0315]

@Gene0315 , as we discussed on discord, let's set a check point to review your NRBC notes added in this repository by 15th June (Sat).

@yungyuc, it’s ok.