Document requirement for custom string types to implement empty() and be constructible from std::string.

Add new option --cpp-str-flex-ctor to construct custom string types not via std::string, but (char * + length).

docs/source/Compiler.md

@@ -75,7 +75,7 @@ Additional options:
 -   `--allow-non-utf8` : Pass non-UTF-8 input through parser and emit nonstandard
     \x escapes in JSON. (Default is to raise parse error on non-UTF-8 input.)
 
--  `--natural-utf8` : Output strings with UTF-8 as human-readable strings. 
+-  `--natural-utf8` : Output strings with UTF-8 as human-readable strings.
      By default, UTF-8 characters are printed as \uXXXX escapes."
 
 -   `--defaults-json` : Output fields whose value is equal to the default value
@@ -120,7 +120,13 @@ Additional options:
 -   `--cpp-ptr-type T` : Set object API pointer type (default std::unique_ptr)
 
 -   `--cpp-str-type T` : Set object API string type (default std::string)
--   T::c_str() and T::length() must be supported.
+    T::c_str(), T::length() and T::empty() must be supported.
+    The custom type also needs to be constructible from std::string (see the
+	--cpp-str-flex-ctor option to change this behavior).
+
+-   `--cpp-str-flex-ctor` : Don't construct custom string types by passing
+    std::string from Flatbuffers, but (char* + length). This allows efficient
+	construction of custom string types, including zero-copy construction.
 
 -   `--object-prefix` : Customise class prefix for C++ object-based API.
 
@@ -168,7 +174,7 @@ Additional options:
     an evolution of. Gives errors if not. Useful to check if schema
     modifications don't break schema evolution rules.
 
--   `--conform-includes PATH` : Include path for the schema given with 
+-   `--conform-includes PATH` : Include path for the schema given with
     `--conform PATH`.
 
 -   `--include-prefix PATH` : Prefix this path to any generated include