new field : ZSTD_frameHeader.frameType

Makes frame type (zstd,skippable) detection more straighforward.
ZSTD_getFrameHeader set frameContentSize=ZSTD_CONTENTSIZE_UNKNOWN to mean "field not present"

doc/zstd_manual.html

@@ -344,9 +344,12 @@ size_t ZSTD_decompressStream(ZSTD_DStream* zds, ZSTD_outBuffer* output, ZSTD_inB
     ZSTD_frameParameters fParams;
 } ZSTD_parameters;
 </b></pre><BR>
+<pre><b>typedef enum { ZSTD_frame, ZSTD_skippableFrame } ZSTD_frameType_e;
+</b></pre><BR>
 <pre><b>typedef struct {
-    unsigned long long frameContentSize;
-    unsigned long long windowSize;   </b>/* can be == frameContentSize */<b>
+    unsigned long long frameContentSize; </b>/* ZSTD_CONTENTSIZE_UNKNOWN means this field is not available. 0 means "empty" */<b>
+    unsigned long long windowSize;       </b>/* can be very large, up to <= frameContentSize */<b>
+    ZSTD_frameType_e frameType;          </b>/* if == ZSTD_skippableFrame, frameContentSize is the size of skippable content */<b>
     unsigned dictID;
     unsigned checksumFlag;
 } ZSTD_frameHeader;