[to #423] add some guideline for triage critical bugs (#503)

Co-authored-by: iosmanthus <dengliming@pingcap.com>
Co-authored-by: ti-srebot <66930949+ti-srebot@users.noreply.github.com>

docs/src/SUMMARY.md

@@ -28,3 +28,4 @@
     - [Region Cache]() (./architecture/region-cache.md)
 
 - [Contribution Guide](./contribution/introduction.md)
+    - [Bug Severity Guidelines](./contribution/bug-severity-guide.md)

docs/src/contribution/bug-severity-guide.md

@@ -0,0 +1,49 @@
+## Bug Severity Guidelines
+
+This is a **working-in-progress** guide about determining defects severity on
+TiKV Java Client according to the impact on the online service. The higher
+effect the defect has on the overall functionality or performance, the higher
+the severity is. There are 4 severity levels:
+
+1. Critical
+2. Major
+3. Moderate
+4. Minor
+
+Each severity is described with examples in the remaining contents.
+
+### Critical Defects
+
+A defect that affects critical data or functionality and leaves users
+with no workaround is classified as a critical defect. These defects are
+labeled with `type/bug` and `severity/critical`, can be found
+[here](https://github.com/tikv/client-java/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3Aseverity%2Fcritical)
+
+Guideline 1. A defect that breaks the API definition is regarded as critical.
+For example:
+
+* [client-java/issues/412](https://github.com/tikv/client-java/issues/412)
+in this defect, gRPC timeout is not set for certain requests, which causes the
+requests can not be terminated as expected when the processing time is too long.
+
+### Major Defects
+
+A defect that affects critical data or functionality and forces users to employ
+a workaround is classified as a major defect. These defects are labeled with
+`type/bug` and `severity/major`, can be found
+[here](https://github.com/tikv/client-java/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3Aseverity%2Fmajor)
+
+### Moderate Defects
+
+A defect that affects non-critical data or functionality and forces users to
+employ a workaround is classified as moderate defect. These defects are labeled
+with `type/bug` and `severity/moderate`, can be found
+[here](https://github.com/tikv/client-java/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3Aseverity%2Fmoderate)
+
+### Minor Defects
+
+A defect that does not affect data or functionality. It does not even need a
+workaround. It does not impact productivity or efficiency. It is merely an
+inconvenience. These defects are labeled with `type/bug` and `severity/minor`,
+can be found
+[here](https://github.com/tikv/client-java/issues?q=is%3Aissue+is%3Aopen+sort%3Aupdated-desc+label%3Aseverity%2Fminor)