Updated FAQ (markdown)

FAQ.md

@@ -1,5 +1,6 @@
 ## Contents
 - [Why can't I do day arithmetic on a `year_month_day`?](#day_arithmetic)
+- [Why is `%A` failing?](#week_day_parse_bug)
 - [Why is `local_t` not a proper clock?](#local_t)
 
 ***
@@ -58,6 +59,33 @@ It would be very easy to add `T& list<T>::operator[](size_t index)`.  But that w
 
 This library continues in that tradition:  The expensive operations are not hidden.
 
+<a name="week_day_parse_bug"></a>
+### Why is `%A` failing?
+
+This program is supposed to work:
+
+```c++
+#include "date/date.h"
+#include <iostream>
+#include <sstream>
+
+int
+main()
+{
+    using namespace date;
+    std::istringstream in{"Sun 2016-12-11"};
+    sys_days tp;
+    in >> parse("%A %F", tp);
+    std::cout << tp << '\n';
+}
+```
+
+But for me it outputs `1970-01-01` and `in.fail()` is true.
+
+_Answer:_ This is a bug in your std::lib `std::time_get` facet.  It _should_ use both `%a` and `%A` identically, and these should both parse either the abbreviated or full `week_day` name in a case-insensitive manner.  Unfortunately some implementations won't do that, resulting in this failure.  These same implementations have the same bug with `%b` and `%B` (month names).
+
+You can work around this bug by compiling with `-DONLY_C_LOCALE`.  This flag restricts you to the "C" locale, but it also avoids use of your `std::time_get` and `std::time_put` facets, instead implementing that logic within this library.
+
 <a name="local_t"></a>
 ### Why is `local_t` not a proper clock?