Add `doc/symbols.md`. [ci skip]

To describe how to use symbols, especially preallocated symbols from C.

doc/guides/symbol.md

+# Symbols
+
+Symbols in `mruby` C source code is represented by `mrb_sym` which is alias of
+`uint32_t`.  Lower 30 bits are used for symbols so that higher 2 bits can be
+used as flags, e.g. `struct mt_elem` in `class.c`.
+
+```C
+struct mt_elem {
+  union mt_ptr ptr;
+  size_t func_p:1;
+  size_t noarg_p:1;
+  mrb_sym key:sizeof(mrb_sym)*8-2;
+};
+```
+
+## C API
+
+We provide following C API for symbols.
+
+### Generate Symbols
+
+#### `mrb_sym mrb_intern(mrb_state*,const char*,size_t)`
+
+Get a symbol from a string.
+
+#### `mrb_sym mrb_intern_check_cstr(mrb_state*,const char*)`
+
+Get a symbol from a NULL terminated (C) string.
+
+#### `mrb_sym mrb_intern_str(mrb_state*,mrb_value)`
+
+Get a symbol from a Ruby string object.
+
+#### `mrb_intern_lit(mrb_state*,const char*)`
+
+Get a symbol from a C string literal. The second argument should be a C string
+literal, otherwise you will get a compilation error. It does not copy C string
+given the fact it's a literal.
+
+#### `mrb_sym mrb_intern_check(mrb_state*,const char*,size_t)`
+
+Get a symbol from a string if the string has been already registered as a
+symbol, otherwise return `0`. We also provide variants `mrb_intern_check_str()`
+(from Ruby string) and `mrb_intern_check_cstr()` (from C string).
+
+#### `const char *mrb_sym_name(mrb_state*,mrb_sym)`
+
+Get a string representation of a symbol as a C string.
+
+#### `const char *mrb_sym_name_len(mrb_state*,mrb_sym,mrb_int*)`
+
+Get a string representation of a symbol, and its length.
+
+## Preallocate Symbols
+
+To save RAM, `mruby` can use compile-time allocation of some symbols. You can
+use following macros to get preallocated symbols by including `mruby/presym.h`
+header.
+
+ *   `MRB_SYM(xor)`    //=> xor    (Word characters)
+ *   `MRB_SYM_B(xor)`  //=> xor!   (Method with Bang)
+ *   `MRB_SYM_Q(xor)`  //=> xor?   (Method with Question mark)
+ *   `MRB_SYM_E(xor)`  //=> xor=   (Method with Equal)
+ *   `MRB_CVSYM(xor)`  //=> @@xor  (Class Variable)
+ *   `MRB_IVSYM(xor)`  //=> @xor   (Instance Variable)
+ *   `MRB_OPSYM(xor)`  //=> ^      (Operator)
+
+For `MRB_OPSYM()`, specify the names corresponding to operators (see
+`MRuby::Presym::OPERATORS` in `lib/mruby/presym.rb` for the names that
+can be specified for it). Other than that, describe only word characters
+excluding leading and ending punctuations.
+
+These macros are converted to static symbol IDs at compile time, unless
+preallocate symbols are disabled by defining `MRB_NO_PRESYM`. In that case,
+these macros are expanded to `mrb_intern_lit` calls, therefore the mruby state
+variable is required. The above macros assume the variable name is `mrb`.  If
+its name is not `mrb`, you need to use macros with `_2` suffix, such as
+`MRB_SYM_2` to specify `mrb_state*` variable.