Document semantic differences from Jam

[Dominic Martinez]

Create a document of Ham's semantic differences from Jam:

• piecemeal is unnecessary (and deprecated)
• together actions only accept one target as a source
• Ham's string parsing
• Ham's operation parsing (e.g. X = Y[3-5] ; $($(X)))
• Variable expansion in actions (no Jam expansion)
• Fixed subscripts
• Evaluation order of action modifiers
• No parent directory modifier
• Pseudotargets are always built
• Lack of member attribute

[Dominic Martinez]

Ham v2 has been moved; closing issues not relevant for v1.