doc: READMEs for remaining "real world" examples

Updates #14

examples/geohash/README.md

@@ -0,0 +1,41 @@
+# geohash
+
+Integer [geohash](https://en.wikipedia.org/wiki/Geohash) encoding.
+
+Refer to ["Geohash in Golang Assembly"](https://mmcloughlin.com/posts/geohash-assembly) for implementation details.
+
+[embedmd]:# (asm.go /func main/ $)
+```go
+func main() {
+	TEXT("EncodeInt", "func(lat, lng float64) uint64")
+	Doc("EncodeInt computes the 64-bit integer geohash of (lat, lng).")
+	lat := Load(Param("lat"), Xv())
+	lng := Load(Param("lng"), Xv())
+
+	MULSD(ConstData("reciprocal180", F64(1/180.0)), lat)
+	onepointfive := ConstData("onepointfive", F64(1.5))
+	ADDSD(onepointfive, lat)
+
+	MULSD(ConstData("reciprocal360", F64(1/360.0)), lng)
+	ADDSD(onepointfive, lng)
+
+	lngi, lati := GP64v(), GP64v()
+	MOVQ(lat, lati)
+	SHRQ(U8(20), lati)
+	MOVQ(lng, lngi)
+	SHRQ(U8(20), lngi)
+
+	mask := ConstData("mask", U64(0x5555555555555555))
+	ghsh := GP64v()
+	PDEPQ(mask, lati, ghsh)
+	temp := GP64v()
+	PDEPQ(mask, lngi, temp)
+	SHLQ(U8(1), temp)
+	XORQ(temp, ghsh)
+
+	Store(ghsh, ReturnIndex(0))
+	RET()
+
+	Generate()
+}
+```