manual split up into multiple files; documented the new concurrency system

doc/manual/about.txt

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+About this document
+===================
+
+**Note**: This document is a draft! Several of Nim's features need more
+precise wording. This manual will evolve into a proper specification some
+day.
+
+This document describes the lexis, the syntax, and the semantics of Nim.
+
+The language constructs are explained using an extended BNF, in
+which ``(a)*`` means 0 or more ``a``'s, ``a+`` means 1 or more ``a``'s, and
+``(a)?`` means an optional *a*. Parentheses may be used to group elements.
+
+``&`` is the lookahead operator; ``&a`` means that an ``a`` is expected but
+not consumed. It will be consumed in the following rule.
+
+The ``|``, ``/`` symbols are used to mark alternatives and have the lowest 
+precedence. ``/`` is the ordered choice that requires the parser to try the 
+alternatives in the given order. ``/`` is often used to ensure the grammar
+is not ambiguous. 
+
+Non-terminals start with a lowercase letter, abstract terminal symbols are in
+UPPERCASE. Verbatim terminal symbols (including keywords) are quoted
+with ``'``. An example::
+
+  ifStmt = 'if' expr ':' stmts ('elif' expr ':' stmts)* ('else' stmts)?
+  
+The binary ``^*`` operator is used as a shorthand for 0 or more occurances
+separated by its second argument; likewise ``^+`` means 1 or more 
+occurances: ``a ^+ b`` is short for ``a (b a)*`` 
+and ``a ^* b`` is short for ``(a (b a)*)?``. Example::
+
+  arrayConstructor = '[' expr ^* ',' ']'
+
+Other parts of Nim - like scoping rules or runtime semantics are only
+described in an informal manner for now.
+