Rename a README file.

Documentation/README.Docker:
   Rename -> README.Docker.md. This file was already in Markdown
   format.

   Adjust heading levels.

   Fix a broken link.

README.md:
    Conform to the above change.

Documentation/README.Docker.md

@@ -1,7 +1,10 @@
+Mercury in Docker
+=================
+
 This file documents using Mercury within a Docker container.
 
 Motivation
-==========
+----------
 
 Docker is a popular way to achieve cross-platform containerisation of
 applications. Due to the rather lengthy compile time of the Mercury compiler
@@ -9,9 +12,9 @@ itself, a way to package the compiler and all of its dependencies is a
 desirable goal.
 
 Building Mercury
-================
+----------------
 
-The [Dockerfile](Dockerfile) is a
+The [Dockerfile](../Dockerfile) is a
 [multi-staged](https://docs.docker.com/develop/develop-images/multistage-build/)
 Dockerfile.
 
@@ -61,7 +64,7 @@ Additionally building the ROTD tarball is supported via:
 ```
 
 Using containerised mmc on the host platform
-============================================
+--------------------------------------------
 
 ```bash
   docker run --rm \
@@ -87,7 +90,7 @@ used:
 ```
 
 Limitations
-===========
+----------
 
 Currently only static linkage is supported, as dynamic linkage searches the
 host for the Mercury runtime library, which might not be installed or not

README.md

@@ -58,7 +58,7 @@ Specific information is contained in individual README files in the
 
   * Other platform information
       * [Cross compilation](Documentation/README.cross.md)
-      * [Docker](Documentation/README.Docker)
+      * [Docker](Documentation/README.Docker.md)
       * [x86](Documentation/README.x86.md)
 
 ## Other information