Add README.md for docker-compose/ext-src directory

Co-Authored-By: alexeymasterov@neon.tech <alexeymasterov@neon.tech>

docker-compose/ext-src/README.md

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+# PostgreSQL Extensions for Testing
+
+This directory contains PostgreSQL extensions used primarily for:
+1. Testing extension upgrades between different Compute versions
+2. Running regression tests with regular users (mostly for cloud instances)
+
+## Directory Structure
+
+Each extension directory follows a standard structure:
+
+- `extension-name-src/` - Directory containing test files for the extension
+  - `test-upgrade.sh` - Script for testing upgrade scenarios
+  - `regular-test.sh` - Script for testing with regular users
+  - Additional test files depending on the extension
+
+## Available Extensions
+
+This directory includes the following extensions:
+
+- `hll-src` - HyperLogLog, a fixed-size data structure for approximating cardinality
+- `hypopg-src` - Extension to create hypothetical indexes
+- `ip4r-src` - IPv4/v6 and subnet data types
+- `pg_cron-src` - Run periodic jobs in PostgreSQL
+- `pg_graphql-src` - GraphQL support for PostgreSQL
+- `pg_hint_plan-src` - Execution plan hints
+- `pg_ivm-src` - Incremental view maintenance
+- `pg_jsonschema-src` - JSON Schema validation
+- `pg_repack-src` - Reorganize tables with minimal locks
+- `pg_roaringbitmap-src` - Roaring bitmap implementation
+- `pg_semver-src` - Semantic version data type
+- `pg_session_jwt-src` - JWT authentication for PostgreSQL
+- `pg_tiktoken-src` - OpenAI Tiktoken tokenizer
+- `pg_uuidv7-src` - UUIDv7 implementation for PostgreSQL
+- `pgjwt-src` - JWT tokens for PostgreSQL
+- `pgrag-src` - Retrieval Augmented Generation for PostgreSQL
+- `pgtap-src` - Unit testing framework for PostgreSQL
+- `pgvector-src` - Vector similarity search
+- `pgx_ulid-src` - ULID data type
+- `plv8-src` - JavaScript language for PostgreSQL stored procedures
+- `postgresql-unit-src` - SI units for PostgreSQL
+- `prefix-src` - Prefix matching for strings
+- `rag_bge_small_en_v15-src` - BGE embedding model for RAG
+- `rag_jina_reranker_v1_tiny_en-src` - Jina reranker model for RAG
+- `rum-src` - RUM access method for text search
+
+## Usage
+
+### Extension Upgrade Testing
+
+The extensions in this directory are used by the `test_extensions_upgrade.sh` script to test upgrading extensions between different versions of Neon Compute nodes. The script:
+
+1. Creates a database with extensions installed on an old Compute version
+2. Creates timelines for each extension
+3. Switches to a new Compute version and tests the upgrade process
+4. Verifies extension functionality after upgrade
+
+### Regular User Testing
+
+For testing with regular users (particularly for cloud instances), each extension directory typically contains a `regular-test.sh` script that:
+
+1. Creates a test database
+2. Installs the extension
+3. Runs regression tests
+
+A note about pg_regress: Since pg_regress attempts to set `lc_messages` for the database by default (which can cause issues), we create databases manually and use the `--use-existing` option to bypass this limitation.
+
+### CI Workflows
+
+Two main workflows use these extensions:
+
+1. **Cloud Extensions Test** - Tests extensions on Neon cloud projects
+2. **Force Test Upgrading of Extension** - Tests upgrading extensions between different Compute versions
+
+## Adding New Extensions
+
+To add a new extension for testing:
+
+1. Create a directory named `extension-name-src` in this directory
+2. Add at minimum:
+   - `test-upgrade.sh` for upgrade testing
+   - `regular-test.sh` for regular user testing
+3. Update the list of extensions in the `test_extensions_upgrade.sh` script if needed