Write release doc for RM.

RELEASING.md

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+# Apache SkyWalking Nginx LUA agent release guide
+
+SkyWalking Nginx LUA agent released through source tar on the [website](https://skywalking.apache.org/downloads/#NginxLUAAgent) 
+and [SkyWalking's LuaRock Module](https://luarocks.org/modules/apache-skywalking/skywalking-nginx-lua).
+
+Release manager could follow this doc to build and upload a release for this agent.
+
+1. Prepare the new `.rockspec` file for the new release. Ref previous and latest files in [here](./rockspec/).
+2. Update the [changelogs](CHANGES.md) for the upcoming release.
+3. Tag through git or GitHub.
+4. Build the source tars with ASC sign and SHA512
+
+```shell
+> export RELEASE_VERSION=x.y.z
+
+> tar czf skywalking-nginx-lua-${RELEASE_VERSION}-src.tgz \
+    --exclude .git \
+    --exclude .DS_Store \
+    --exclude .github \
+    --exclude .gitignore \
+    --exclude .gitmodules \
+    --exclude .mvn/wrapper/maven-wrapper.jar \
+    skywalking-nginx-lua
+
+> gpg --armor --detach-sig skywalking-nginx-lua-${RELEASE_VERSION}-src.tgz
+
+> shasum -a 512 skywalking-nginx-lua-${RELEASE_VERSION}-src.tgz > skywalking-nginx-lua-${RELEASE_VERSION}-src.tgz.sha512
+
+```
+
+5. Upload `*-src.tgz`, `*-src.tgz.asc` and `*-src.tgz.sha512` to SVN `https://dist.apache.org/repos/dist/release/skywalking/nginx-lua/${RELEASE_VERSION}`
+
+6. Call for vote through `dev@skywalking.apache.org` mailing list.
+
+```
+Hi all,
+
+This is a call for vote to release Apache SkyWalking Nginx LUA version ${RELEASE_VERSION}.
+
+Release notes:
+
+ * https://github.com/apache/skywalking-nginx-lua/blob/v${RELEASE_VERSION}/CHANGES.md
+
+Release Candidate:
+
+ * https://dist.apache.org/repos/dist/dev/skywalking/nginx-lua/${RELEASE_VERSION}/
+ * sha512 checksums
+   - xxx  skywalking-nginx-lua-${RELEASE_VERSION}-src.tgz
+
+Release Tag :
+
+ * v${RELEASE_VERSION}
+
+Release CommitID :
+
+ * https://dist.apache.org/repos/dist/dev/skywalking/nginx-lua/${COMMIT_ID}
+
+Keys to verify the Release Candidate :
+
+ * https://dist.apache.org/repos/dist/release/skywalking/KEYS
+
+
+Voting will start now (Date) and will remain open for at least
+72 hours, Request all PMC members to give their vote.
+[ ] +1 Release this package.
+[ ] +0 No opinion.
+[ ] -1 Do not release this package because....
+```
+
+7. If the vote passed with at least +1 binding(s) and more +1 binding(s) than -1 binding(s), the vote pass.
+
+8. Upload the rockspec to LuaRocks
+
+> luarocks upload skywalking-nginx-lua-${RELEASE_VERSION}-0.rockspec
+
+9. Move the source tar from svn dev folder to the release folder. 
+
+> svn mv https://dist.apache.org/repos/dist/dev/skywalking/nginx-lua/${RELEASE_VERSION} https://dist.apache.org/repos/dist/release/skywalking/nginx-lua
+
+10. Update website event page accordingly.