Use latest tag for versioning. (#160)

Signed-off-by: Andrei Warkentin <[email protected]>

Makefile

@@ -13,7 +13,7 @@
 # the Doc Template for RISC-V Extensions.
 
 DATE ?= $(shell date +%Y-%m-%d)
-VERSION ?= v0.0.1
+VERSION ?= $(shell git describe --tags)
 REVMARK ?= \"This document is in development. Assume everything can change. See http://riscv.org/spec-state for details.\"
 DOCKER_RUN := docker run --rm -v ${PWD}:/build -w /build \
 riscvintl/riscv-docs-base-container-image:latest