update docs, rename docs folder, add error condition

docs/format/index.md

@@ -65,7 +65,7 @@ Strings are referenced by pointing to the beginning of their length field. The s
 ### Series
 
 The section contains a sequence of series that hold the label set of the series as well as its chunks within the block. The series are sorted lexicographically by their label sets.  
-The file offset to the beginning of a series serves as the series' ID in all subsequent references. Thereby, a sorted list of series IDs implies a lexicographically sorted list of series label sets.
+Each series section is aligned to 16 bytes. The ID for a series is the `offset/16`. This serves as the series' ID in all subsequent references. Thereby, a sorted list of series IDs implies a lexicographically sorted list of series label sets. 
 
 ```
 ┌───────────────────────────────────────┐

index/index.go

@@ -575,12 +575,12 @@ func NewReader(b ByteSlice, version int) (*Reader, error) {
 	return newReader(b, nil, version)
 }
 
-func NewReaderV1(b ByteSlice, c io.Closer, version int) (*Reader, error) {
-	return newReader(b, c, version)
-}
-
 // NewFileReader returns a new index reader against the given index file.
 func NewFileReader(path string, version int) (*Reader, error) {
+	if version != 1 && version != 2 {
+		return nil, errors.Errorf("unexpected file version %d", version)
+
+	}
 	f, err := fileutil.OpenMmapFile(path)
 	if err != nil {
 		return nil, err