Provide a mechanism to compose type definitions

This extends the syntax of the --type-add flag to allow including the globs of
other already defined types.

Fixes #83.

doc/rg.1

@@ -397,9 +397,38 @@ inside of ripgrep.
 Note that this must be passed to every invocation of rg.
 Type settings are NOT persisted.
 .RS
+.IP
+.nf
+\f[C]
+\ \ Example:\ `rg\ \-\-type\-add\ \[aq]foo:*.foo\[aq]\ \-tfoo\ PATTERN`
+\f[]
+.fi
 .PP
-Example:
-\f[C]rg\ \-\-type\-add\ \[aq]foo:*.foo\[aq]\ \-tfoo\ PATTERN\f[]
+\-\-type\-add can also be used to include rules from other types with
+the special include directive.
+The include directive permits specifying one or more other type names
+(separated by a comma) that have been defined and its rules will
+automatically be imported into the type specified.
+For example, to create a type called src that matches C++, Python and
+Markdown files, one can use:
+.IP
+.nf
+\f[C]
+\ \ `\-\-type\-add\ \[aq]src:include:cpp,py,md\[aq]`
+\f[]
+.fi
+.PP
+Additional glob rules can still be added to the src type by using the
+\-\-type\-add flag again:
+.IP
+.nf
+\f[C]
+\ \ `\-\-type\-add\ \[aq]src:include:cpp,py,md\[aq]\ \-\-type\-add\ \[aq]src:*.foo\[aq]`
+\f[]
+.fi
+.PP
+Note that type names must consist only of Unicode letters or numbers.
+Punctuation characters are not allowed.
 .RE
 .TP
 .B \-\-type\-clear \f[I]TYPE\f[] ...