docs: add compat docs

Signed-off-by: Bence Csati <bence.csati@axoflow.com>

Author: csatib02

README.compat.md

@@ -0,0 +1,77 @@
+# Compat
+
+## Mixed Elasticsearch Version Environments
+
+### Compatibility Matrix
+
+| Elasticsearch Gem | ES 7.x Server | ES 8.x Server | ES 9.x Server |
+|-------------------|---------------|---------------|---------------|
+| gem 7.17.x        | ✅ Works      | ✅ Works      | ❌ No         |
+| gem 8.15.x        | ✅ Works      | ✅ Works      | ❌ No         |
+| gem 9.1.x + patch | ✅ With config| ✅ With config| ✅ Works      |
+
+### Configuration Options
+
+#### force_content_type
+
+Type: String
+Default: nil (auto-detect)
+Valid values: "application/json", "application/x-ndjson"
+Manually override the Content-Type header. Required when using elasticsearch gem 9.x with ES 7.x or 8.x servers.
+
+```ruby
+<match **>
+  @type elasticsearch
+  force_content_type application/json
+</match>
+```
+
+#### ignore_version_content_type_mismatch
+
+Type: Bool
+Default: false
+Automatically fallback to application/json if Content-Type version mismatch occurs. Enables seamless operation across mixed ES 7/8/9 environments.
+
+Example:
+
+```ruby
+<match **>
+  @type elasticsearch
+  force_content_type application/json
+  ignore_version_content_type_mismath true
+</match>
+```
+
+### Recommended Configuration
+
+#### For ES 7/8 environments (Recommended)
+
+Use elasticsearch gem 8.x - works with both versions, no configuration needed:
+
+```ruby
+# Gemfile
+gem 'elasticsearch', '~> 8.15.0'
+
+# fluent.conf
+<match **>
+  @type elasticsearch
+  hosts es7:9200,es8:9200
+  # No special config needed!
+</match>
+```
+
+#### For gem 9.x with ES 7/8 (Not recommended, but supported)
+
+```ruby
+# Gemfile
+gem 'elasticsearch', '~> 9.1.0'
+
+# fluent.conf
+<match **>
+  @type elasticsearch
+  hosts es7:9200,es8:9200
+  
+  # REQUIRED: Force compatible content type
+  force_content_type application/json
+</match>
+```