Change log updates

ChangeLog.md

@@ -1,5 +1,62 @@
 ## Changes Between 1.1.0 and 2.0.0
 
+### Metrics Registries
+
+`metrics-clojure` 1.x maintained a metrics registry in a dynamic var.
+This approach makes the library a little easier for beginners but
+also much harder to use in more sophisticated cases, e.g. in concurrent
+applications or those that use a Component-like approach to
+program structure.
+
+As such, `metrics-clojure` 2.0+ makes metrics registry a required
+explicit argument to most functions in the API:
+
+``` clojure
+(require '[metrics.meters :as meters])
+
+;; with 1.x
+(meters/rate-mean)
+(meters/mark! 10)
+
+;; with 2.0
+(let [m (meters/meter ["test" "meters" "test-rate-mean-update-multiple"])]
+  (meters/rate-mean m)
+  (meters/mark! m 10))
+```
+
+The library maintains a default registry in `metrics.core/default-registry`
+which tries to keep the `1.x` API as functional as possible but using
+your own registry is encouraged.
+
+To instantiate a registry, use `metrics.core/new-registry`:
+
+``` clojure
+(require '[metrics.core :as mtr])
+
+(mtr/new-registry)
+```
+
+See [GH #19](https://github.com/sjl/metrics-clojure/issues/19) for
+discussion.
+
+### defgauge Restricted to Functions Only
+
+In `metrics-clojure` 1.x, `metrics.gauges/defgauge` could accept
+a function or a bunch of forms (body). In 2.0, it only accepts
+a function. This is in part due to the new API structure but also
+make the API more straightforward and works much better with explicit
+registry management now advocated by the library.
+
+
+### Nanoseconds Precision in Timers
+
+Metrics 3.0 uses nanoseconds precision in timers.
+
+
+### Upgrade to Metrics 3.0
+
+Metrics 3.0 is now used internally by the library.
+
 ### Clojure 1.3 No Longer Supported
 
 Clojure 1.3 is no longer supported by the library.