middleware: added varinterval

middleware/varinterval/README.md

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+## Announce Interval Variation Middleware
+
+This package provides the announce middleware `varinterval` which randomizes the announce interval.
+
+### Functionality
+
+This middleware will choose random announces and modify the `interval` and `min_interval` fields.
+A random number of seconds will be added to the `interval` field and, if desired, also to the `min_interval` field.
+
+Note that if a response is picked for modification and `min_interval` should be changed as well, both `interval` and `min_interval` will be modified by the same amount.
+
+### Use Case
+
+Use this middleware to avoid recurring load spikes on the tracker.
+By randomizing the announce interval, load spikes will flatten out after a few cycles.
+
+### Configuration
+
+This middleware provides the following parameters for configuration:
+
+- `modify_response_probability` (float, >0, <= 1) indicates the probability by which a response will be augmented with random peers.
+- `max_increase_delta` (int, >0) sets an upper boundary (inclusive) for the amount of seconds added.
+- `modify_min_interval` (boolean) whether to modify the `min_interval` field as well.
+
+An example config might look like this:
+
+    chihaya:
+      tracker:
+        announce_middleware:
+          - name: varinterval
+            config:
+              modify_response_probability: 0.2
+              max_increase_delta: 60
+              modify_min_interval: true