describe OCRD_PROFILE{,_FILE} in README

Author: kba

README.md

@@ -19,6 +19,7 @@
 * [Command line tools](#command-line-tools)
 	* [`ocrd` CLI](#ocrd-cli)
 	* [`ocrd-dummy` CLI](#ocrd-dummy-cli)
+* [Configuration](#configuration)
 * [Packages](#packages)
 	* [ocrd_utils](#ocrd_utils)
 	* [ocrd_models](#ocrd_models)
@@ -81,6 +82,19 @@ supported flags, options and arguments.
 
 A minimal [OCR-D processor](https://ocr-d.de/en/user_guide#using-the-ocr-d-processors) that copies from `-I/-input-file-grp` to `-O/-output-file-grp`
 
+## Configuration
+
+Almost all behaviour of the OCR-D/core software is configured via CLI options and flags, which can be listed with the `--help` flag that all CLI support.
+
+Some parts of the software are configured via environement variables:
+
+* `OCRD_METS_CACHING`: If set to `true`, access to the METS file is cached, speeding in-memory search and modification.
+* `OCRD_PROFILE`: This variable configures the built-in CPU and memory profiling. If empty, no profiling is done. Otherwise expected to contain any of the following tokens:
+  * `CPU`: Enable CPU profiling of processor runs
+  * `RSS`: Enable RSS memory profiling
+  * `PSS`: Enable proportionate memory profiling
+* `OCRD_PROFILE_FILE`: If set, then the CPU profile is written to this file for later peruse with a analysis tools like [snakeviz](https://jiffyclub.github.io/snakeviz/)
+
 ## Packages
 
 ### ocrd_utils