Add limitation and future scope and concerns for fixed fee spec. - hid9

[Vishwas1]

• limitation : Since, SSI messages incur fixed cost and thus independent of gas consumption, they are NOT allowed to be grouped together with transaction messages of other SDK modules.
• Future scope : we may introduce fee for each VM method
• security concern: an attacker can create a did with 1000 vms and do the transaction and will end up paying same fee as any other user who has just 1 vm in a did doc - probably can exploit the system. This does not seems to be fair for validators who are verifying the did with 1000 vm getting same fee paid as validators verifying did with 1 vm