doc: Added serial recovery documentation

Added description on what is the serial recovery protocol
in general.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>
Signed-off-by: Anna Kielar <anna.kielar@nordicsemi.no>

fix
diff --git a/docs/index.md b/docs/index.md
index c6f5e82..591fb00 100644
--- a/docs/index.md
+++ b/docs/index.md
@@ -38,6 +38,7 @@
 - [Encrypted images](encrypted_images.md)
 - [imgtool](imgtool.md) - image signing and key management
 - [ECDSA](ecdsa.md) - information about ECDSA signature formats
+- [Serial Recovery](serial_recovery.md) - MCUmgr as the serial recovery protocol
 - Usage instructions:
   - [Zephyr](readme-zephyr.md)
   - [Apache Mynewt](readme-mynewt.md)