convert Changes manuals book into markdown

INSTALL.md

@@ -399,8 +399,7 @@ GAP Reference manual.
 
 The manual is also available in pdf format. In the full distribution these
 files are included in the directory `gap-4.X.Y/doc` in the subdirectories
-`tut` (a beginner's tutorial), `ref` (the reference manual) and `changes`
-(changes from earlier versions).
+`tut` (a beginner's tutorial) and `ref` (the reference manual).
 
 If you want to use these manual files with the help system from your GAP
 session you may check (or make sure) that your system provides some

doc/README.md

@@ -1,15 +1,16 @@
-GAP has three main GAP manuals (they are called "main" to distinguish them from package 
+GAP has two main GAP manuals (they are called "main" to distinguish them from package 
 manuals that are maintained by the authors of the respective packages):
 * **GAP Tutorial**
 * **GAP Reference Manual**
-* **Changes from Earlier Versions**
 
 These manuals are written in GAPDoc format, provided by the 
 [GAPDoc package](http://www.math.rwth-aachen.de/~Frank.Luebeck/GAPDoc/index.html). 
-Their source is contained in the directories `doc/tut`, `doc/ref` and `doc/changes` 
+Their source is contained in the directories `doc/tut` and `doc/ref`
 respectively. Some documentation is also stored in the library files to be kept 
 close to the code it describes. It is included in the manual using the mechanism 
-documented [here](https://www.gap-system.org/Manuals/pkg/GAPDoc-1.5.1/doc/chap4.html).
+documented [here](https://www.gap-system.org/Manuals/pkg/GAPDoc-1.6.2/doc/chap4.html).
+
+There is also a document describing [**Changes from Earlier Versions**](Changes.md).
 
 The official GAP distribution includes all documentation, so there is no need to 
 build it after GAP installation. However, if you need to build the development 
@@ -21,10 +22,10 @@ steps:
   the latest GAP release).
 * build manuals by calling `make doc`
 
-This will build all three manuals. Each of them will be built twice to ensure that 
+This will build the manuals. Each of them will be built twice to ensure that 
 cross-references between manuals are resolved.
 
-Each of these three directories contains a file `makedocrel.g`. You may read it into 
+Both of these directories contain a file `makedocrel.g`. You may read it into 
 GAP if you want to build faster just one manual to see how your changes look like, 
 or if GAPDoc reports an error in the XML code that you want to debug. Then you may 
 call `make doc` as a final check at a later stage.