Documentation update

This is update of documentation draft before code implementation.

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+Built-in commands
+=================
+Here is full list of all build in commands including its description and sometime
+usage examples. For more information see syntax documentation.
+
+All build in commands are defined in lover case as they are presented here, but
+also in upper case. Also you can change initial character, we use here default one
+(`$`).
+
+###	$include(PATH)
+Includes file content to its place. `PATH` is system path relative to current file
+folder or absolute path.
+
+With this command you can nest any other file content and this content. Note that
+because of evaluation principles, content can be evaluated multiple times. If it
+is not what you want, common idiom would be use:
+```
+$limit(0)$include(file)$endlimit
+```
+
+### $load(...)
+This loads additional modules. sgp searches with precedence for modules in file
+directory and in paths defined by environment variable `SGPMODULES`. `NAME` is in
+such case relative path to module, but also absolute path can be used.
+
+Be aware that modules stays loaded even after current file preprocessing is
+finished. They are not unloaded automatically after return from include.
+
+For more information see module documentation.
+
+### $unload(...)
+Serves as opposite to `load` command. It is as it says unloading modules. But it
+is not paired with load.
+
+Without any argument `unload` will unload all loaded modules.
+
+### $limit(NUM)
+This command limits number of evaluation cycles for some text (see evaluation
+documentation).  After this command all text will be evaluated only `NUM` times.
+So `NUM` should be number. Passing empty string results in disabled evaluation
+until end of limit command. Default value if not limit command is used is `inf`,
+that means no limit.  Limit can be ended using `endlimit` command.
+
+Because of how limit works. Most likely you will use limit without any argument or
+with `inf`, or with `1`. Lets see once again what every one does. No argument is
+same as `0` so no evaluation is done until `endlimit` command. `inf` removes any
+limit and allows any number of nested evaluations. `1` specifies that evaluation
+should be done only once.
+
+You can nest limits together. This results to situation when content is evaluated
+limited times but some part of it that is wrapped in another limit block will be
+evaluated few times more without sucking up its root limit.
+
+### $endlimit
+This ends previous evaluation limit. Text after this command is not limited by
+that.
+
+### $"
+### $'
+This is almost same command as `$"`. It has same behavior as if you would write:
+```
+$"$limit $Text $endlimit$"
+```
+Where result would be:
+```
+ $Text 
+```
+So its also limits evaluation.
+
+### $set(VAR,VALUE)
+### $define(VAR,...)
+### $enddefine
+### $undefine(VAR)
+### $math(EQ)
+### $if(COND)
+### $ifdef(VAR)
+### $ifndef(VAR)
+### $elif(COND)
+### $else
+### $endif
+### $for(VAR,COND,EQ)
+### $endfor
+### $while(COND)
+### $endwhile
+### $switch(VAR)
+### $case(VAL)
+### $default
+### $break
+### $continue
+### $exec(EXEC,...)
+Calls executable `EXEC` with rest of arguments as arguments for executed program.
+Standard output from this program is than included to same place as `exec`.
+
+### $shell(CMD)
+This is same as exec, except that instead of defining executable and command line
+arguments separately they are defined together in `CMD` argument. In fact you can
+specify any shell command in `CMD`.
+
+### $error(TEXT)
+`TEXT` is printed to error output and spg is terminated.
+
+### $warning(TEXT)
+`TEXT` is printed to error output, but sgp continuous preprocessing text.
+
+### $info(TEXT)
+Simply prints `TEXT` to standard output. If standard output is used as output of
+result, info is printed to error output.