doc: local development instructions

README.md

 # nexus_semantic_logger
 
 Configures a [semantic_logger](https://rubygems.org/gems/rails_semantic_logger) as required for NexusMods components.
+
+# Local gem development
+
+Steps to run this gem from local sources in one the nexus 'staged build' rails components:
+
+## Copy gem sources to component
+
+```
+cd ~/legacy/users
+cp -r ../nexus_semantic_logger .
+```
+
+## Adjust component Dockerfile to include gem sources
+
+Within stage 1, append a COPY after the Gemfile copy:
+
+```
+COPY --chown=nexus:nexus Gemfile* ./
+COPY --chown=nexus:nexus nexus_semantic_logger/ ./nexus_semantic_logger/
+```
+
+Within stage 2, append a COPY after the bundle copy:
+
+```
+COPY --from=stage1 /usr/local/bundle /usr/local/bundle
+COPY --from=stage1 /app/nexus_semantic_logger/ /app/nexus_semantic_logger/
+```
+
+## Adjust Gemfile to use local path
+
+```
+gem 'nexus_semantic_logger', :path => "/app/nexus_semantic_logger"
+