docs: document the tr_languages langopts

docs/README.md

 - Creating and Editing Languages and Voices
   - [Adding or Improving a Language](add_language.md)
     - [Text to Phoneme Translation](dictionary.md)
+    - [Numbers](numbers.md)
   - [Voice Files](voices.md)
     - [MBROLA Voices](mbrola.md)
     - [Phoneme Tables](phontab.md)

docs/numbers.md

+# Numbers
+
+- [Language Options](#language-options)
+
+## Language Options
+
+The following properties in `tr_languages.c` control how numbers behave for a
+language:
+
+    tr->langopts.numbers
+    tr->langopts.numbers2
+
+These controls how numbers are pronounced.
+
+If `numbers` is set to `0` (the default value), numbers will not be pronounced.
+Setting it to `1` will enable number pronunciation using the dictionary rules.
+
+    tr->langopts.max_digits
+
+This is the maximum number of digits that can be spoken by the language. If the
+number of digits is larger than this, the number is spoken digit by digit.
+
+    tr->langopts.break_numbers
+
+This controls how digits are grouped for speaking large numbers. The default is
+to group into 3 digit groups (thousand, million, billion, trillion, etc.).
+
+    tr->langopts.thousands_sep
+
+The character used to separate digit groups. The default is `,`.
+
+    tr->langopts.decimal_sep
+
+The character used to separate the integer and decimal parts of a real number.
+The default is `.`.
+