Fix Issue #11384 - Explaining how to load external backends in the documentation
diff --git a/docs/templates/backend.md b/docs/templates/backend.md
index 256659b578c..c671492e201 100644
--- a/docs/templates/backend.md
+++ b/docs/templates/backend.md
@@ -45,6 +45,20 @@ KERAS_BACKEND=tensorflow python -c "from keras import backend"
 Using TensorFlow backend.
 ```
 
+In Keras it is possible to load more backends than `"tensorflow"`, `"theano"`, and `"cntk"`. Keras can use external backends as well, and this can be performed by changing the `keras.json` configuration file, and the `"backend"` setting. Suppose you have a Python module called `my_module` that you wanted to use as your external backend. The `keras.json` configuration file would be changed as follows:
+
+```
+{
+    "image_data_format": "channels_last",
+    "epsilon": 1e-07,
+    "floatx": "float32",
+    "backend": "my_package.my_module"
+}
+```
+An external backend must be validated in order to be used, a valid backend must have the following functions: `placeholder`, `variable` and `function`.
+
+If an external backend is not valid due to missing a required entry, an error will be logged notifying which entry/entries are missing.
+
 ----
 
 ## keras.json details

### Summary
Created additional documentation for the "Switching from one backend to another" section of the documentation, including an example of an edited keras.json configuration file.

### Related Issues
Solves issue #11384 

### PR Overview

- [ ] This PR requires new unit tests [y/n] (make sure tests are included)
- [x] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
- [x] This PR is backwards compatible [y/n]
- [ ] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)
