Readme updates

[Prof9]

• readme.txt was removed as it's fairly out of date.
• Section 4.3 Labels:
  • Has a more detailed explanation of how labels work, including some semantics about identifiers and symbols.
  • Some intricacies with static labels and macros are explained.
  • Added explanation of .func labels and how it affects the symfile.
• Section 4.4 equ:
  • Cleaned up a bit.
  • Explains global/static/local equ syntax and differences a bit more explicitly.
• Section 4.5 Expression parser:
  • A subsection Value types is added, explaining some intricacies with floats and strings.
  • defined() was added to the subsection Built-in functions.
  • Some clarifications to existing functions were added.
• Section 4.7 Strings:
  • Renamed String encoding.
  • Rewrote a bit and moved some parts of it to subsection 4.5 Value types.
• Section 4.8 Areas:
  • Added the missing feature where you can specify a fill byte for the area.
• Section 5.1 General directives:
  • Added missing .openfile alias and an explicit example for .open (since quotation marks were not required for earlier ARMIPS versions, I've seen some people be confused by it).
  • For .open, .create, .close, .org noted that local labels and equs are discarded.
  • Added the missing feature where you can specify a fill byte for .align.
  • Added a note that .align without specifying num defaults to .align 4.
  • Added missing data directive aliases.
  • Added info on how No$gba semantics change certain data directives.
  • Clarified that data directives use ASCII encoding for strings.
  • For .ifdef, changed "identifier" parameter name to "symbol" and clarified that this effectively refers to a label.
  • For .definelabel, added undefined behavior when combined with org(), . etc.
  • Added a subsection Function labels which details use of the .func/.endfunc directives.
  • Added a subsection Error on warning which details use of the .erroronwarning directive.
  • Added a subsection Relative paths which details use of the .relativeinclude directive.
  • Added a subsection No$gba semantics which details use of the .nocash directive.
  • Added a subsection Enable/disable symfile writing which details use of the .sym directive.
  • Some other small clarifications/changes.
• Section 6.3 User defined macros:
  • Fixed part that said global labels aren't affected by label rewriting, also added static labels.
  • Fixed how the rewritten macro label is formatted.
  • Added info on passing a label identifier into a macro as parameter to be defined inside the macro.

[Prof9]

• Section 5.1 General directives:
  • Added a note that .align without specifying num defaults to .align 4.

[Prof9]

Replaced any instance of the word "area" where it refers to "the area where local labels and equs can be used" to "scope" instead, to avoid confusion with the .area directive.