docs: add copyfile and linkfile elements description

The "copyfile" element is available since 2009 and
have been used in every Android manifest; the "linkfile"
element is available since 2014.
Now it's a good time to add both to the documentation

Change-Id: Ia987edf5f69a006235fbd3f33b744e9794a6d964
Signed-off-by: Ruslan Bilovol <ruslan.bilovol@gmail.com>

docs/manifest-format.txt

@@ -50,7 +50,9 @@ following DTD:
     <!ATTLIST url              CDATA #REQUIRED>
 
     <!ELEMENT project (annotation*,
-                       project*)>
+                       project*,
+                       copyfile?,
+                       linkfile?)>
     <!ATTLIST project name        CDATA #REQUIRED>
     <!ATTLIST project path        CDATA #IMPLIED>
     <!ATTLIST project remote      IDREF #IMPLIED>
@@ -68,6 +70,14 @@ following DTD:
     <!ATTLIST annotation value CDATA #REQUIRED>
     <!ATTLIST annotation keep  CDATA "true">
 
+    <!ELEMENT copyfile (EMPTY)>
+    <!ATTLIST src value  CDATA #REQUIRED>
+    <!ATTLIST dest value CDATA #REQUIRED>
+
+    <!ELEMENT linkfile (EMPTY)>
+    <!ATTLIST src value  CDATA #REQUIRED>
+    <!ATTLIST dest value CDATA #REQUIRED>
+
     <!ELEMENT extend-project>
     <!ATTLIST extend-project name CDATA #REQUIRED>
     <!ATTLIST extend-project path CDATA #IMPLIED>
@@ -285,6 +295,21 @@ prefixed with REPO__.  In addition, there is an optional attribute
 "false".  This attribute determines whether or not the annotation will
 be kept when exported with the manifest subcommand.
 
+Element copyfile
+----------------
+
+Zero or more copyfile elements may be specified as children of a
+project element. Each element describes a src-dest pair of files;
+the "src" file will be copied to the "dest" place during 'repo sync'
+command.
+"src" is project relative, "dest" is relative to the top of the tree.
+
+Element linkfile
+----------------
+
+It's just like copyfile and runs at the same time as copyfile but
+instead of copying it creates a symlink.
+
 Element remove-project
 ----------------------