docs: describe log levels

docs/code_contribution_guidelines.md

@@ -12,6 +12,7 @@
 4.7. [Protobuf Compilation](#Protobuf)<br />
 4.8. [Additional Style Constraints On Top of gofmt](ExtraGoFmtStyle)<br />
 4.9. [Pointing to Remote Dependant Branches in Go Modules](ModulesReplace)<br />
+4.10. [Use of Log Levels](#LogLevels)<br />
 5. [Code Approval Process](#CodeApproval)<br />
 5.1. [Code Review](#CodeReview)<br />
 5.2. [Rework Code (if needed)](#CodeRework)<br />
@@ -166,6 +167,8 @@ A quick summary of test practices follows:
   [`networkHarness`framework](https://github.com/lightningnetwork/lnd/blob/master/lntest/harness.go)
   contained within `lnd`. For example integration tests, see
   [`lnd_test.go`](https://github.com/lightningnetwork/lnd/blob/master/lnd_test.go#L181). 
+- The itest log files are automatically scanned for `[ERR]` lines. There
+  shouldn't be any of those in the logs, see [Use of Log Levels](#LogLevels).
 
 Throughout the process of contributing to `lnd`, you'll likely also be
 extensively using the commands within our `Makefile`. As a result, we recommend
@@ -512,6 +515,16 @@ Here's an example replacing the `lightning-onion` version checked into `lnd` wit
  go mod edit -replace=github.com/lightningnetwork/lightning-onion@v0.0.0-20180605012408-ac4d9da8f1d6=github.com/roasbeef/lightning-onion@2e5ae87696046298365ab43bcd1cf3a7a1d69695
 ```
 
+<a name="LogLevels" />
+
+#### 4.10 Use of Log Levels
+
+There are six log levels available: `trace`, `debug`, `info`, `warn`, `error` and `critical`.
+
+Only use `error` for internal errors that are never expected to happen during
+normal operation. No event triggered by external sources (rpc, chain backend,
+etc) should lead to an `error` log.
+
 <a name="CodeApproval" />
 
 ### 5. Code Approval Process