Added installation docs about Blackfire (#4)

* Added installation docs about Blackfire

* Rebase fix

Author: Nyholm

Readme.md

@@ -6,6 +6,12 @@ a limit of 5 layers per Lambda.
 
 We are happy to get contributions for other extensions. Sky is the limit! (And also your knowledge with Docker...)
 
+## Install and configure
+
+```cli
+composer require bref/extra-php-extensions
+```
+
 ```yaml
 # serverless.yml
 service: app
@@ -33,7 +39,7 @@ functions:
 extension=/opt/bref-extra/amqp.so
 ```
 
-## Available layers
+### Available layers
 
 | Name | Serverless config (php 7.4) | php.ini config |
 | ---- | ----------------------------| -------------- |
@@ -46,7 +52,42 @@ extension=/opt/bref-extra/amqp.so
 
 Note that the "Memcached" layer provides both extension for [Memcache](https://pecl.php.net/package/memcache) and [Memcached](https://pecl.php.net/package/memcached). 
 
-## Deploy new versions
+### Blackfire installation
+
+The Blackfire layer only have probe installed. You still need to install the agent. 
+The agent is installed on a separate server (not a lambda function). The very smallest 
+EC2 instance is sufficient to runt the Blackfire agent.
+
+Create a `blackfire.ini` file for your lambda function where you load the extension 
+and modify the `agent_socket`.
+
+```ini
+extension=/opt/bref-extra/blackfire.so
+
+blackfire.agent_socket = tcp://ip-172-40-40-40.eu-central-1.compute.internal:8307
+blackfire.agent_timeout = 0.25
+```
+
+Then modify your [agent config](https://blackfire.io/docs/reference-guide/configuration#agent-configuration) 
+to make sure you are listening to `tcp://0.0.0.0:8307`.  
+
+## For contributors and maintainers
+
+### Creating a new layer
+
+The idea is to start from bref/build-php-XX, install all libraries and extensions
+you want, then move all related files to `/opt`. Those files will be available in
+the same same location on the Lambda.
+
+Note that one can't just move files/libraries around. Most of them are expected to
+be in their "standard" location.  
+
+1. Create a new folder in `layers` and name it to your extension name.
+2. Add your Dockerfile
+3. Update .travis.yml to include your extension
+4. Update the table in the readme
+
+### Deploy new versions
 
 ```
 make publish