fix version in v2/gax.go, add RELEASING.md (#76)

This commit will be tagged v2.0.3.
diff --git a/RELEASING.md b/RELEASING.md
new file mode 100644
index 0000000..6e3654f
--- /dev/null
+++ b/RELEASING.md
@@ -0,0 +1,9 @@
+# Releasing
+
+1. Make changes.
+2. Determine next version. For the sake of this list, let's assume 1.2.3.
+3. Update `gax.go` with `const Version = "1.2.3"`. If v2, do the same to `v2/gax.go`.
+4. `git checkout -b my_branch && git commit && git push my_remote my_branch`
+5. Get it reviewed and merged.
+6. `git checkout master && git tag v1.2.3 && git push origin v1.2.3`
+7. Update the [releases page on GitHub](https://github.com/googleapis/gax-go/releases)
diff --git a/v2/gax.go b/v2/gax.go
index 1ba4629..8040dcb 100644
--- a/v2/gax.go
+++ b/v2/gax.go
@@ -36,4 +36,4 @@
 package gax
 
 // Version specifies the gax-go version being used.
-const Version = "2.0.1"
+const Version = "2.0.3"