Restores the information on the api field

As requested by @jonathanstowe and @ugexe

doc/Language/modules.pod6

@@ -6,12 +6,12 @@
 
 =head1 Creating and using modules
 
-A module is usually a source file or set of source files that expose Perl 6
-constructs.
+A module is usually a source file or set of source files that expose
+Perl 6 constructs.
 
-N<Technically a
-module is a set of I<compunits> which are usually files but could come from
-anywhere as long as there is a I<compunit repository> that can provide it. See
+N<Technically a module is a set of I<compunits> which are usually files
+but could come from anywhere as long as there is a I<compunit
+repository> that can provide it. See
 L<S11|https://design.perl6.org/S11.html>.>
 
 Modules are typically packages (L<classes|/language/objects#Classes>,
@@ -24,21 +24,23 @@ mean "module" as a set of source files in a namespace.
 
 =head2 Looking for and installing modules.
 
-L<C<zef>|https://github.com/ugexe/zef> is the application using for installing
-modules in Perl 6. Modules are listed in
-L<the Perl 6 ecosystem|https://modules.perl6.org>
-and can be searched there or from the
-command line using C<zef search>
+L<C<zef>|https://github.com/ugexe/zef> is the application used for
+installing modules in Perl 6. Modules are listed in L<the Perl 6
+ecosystem|https://modules.perl6.org> and can be searched there or from
+the command line using C<zef search>
 
 =for code :lang<shell>
     zef search WWW
 
-will return a list of modules that includes WWW in their name, for instance. Then,
+will return a list of modules that includes WWW in their name, for
+instance. Then,
 
 =for code :lang<shell>
     zef install WWW
 
-will install the module with that particular name, if it is not already installed.
+will install the module with that particular name, if it is not already
+installed. N<If it's installed it will reinstall only if the version of
+the module is newer than the one installed>
 
 
 =head2 Basic structure
@@ -677,6 +679,7 @@ Make your X<C<META6.json>|META6.json> file look something like this:
     {
         "perl" : "6.c",
         "name" : "Vortex::TotalPerspective",
+        "api"  : "1",
         "auth" : "github:SomeAuthor",
         "version" : "0.0.1",
         "description" : "Wonderful simulation to get some perspective.",
@@ -714,10 +717,18 @@ The C<name> key is compulsory, and C<zef> will fail if you do not include it.
 Even if you have created a META6.json file just to express the dependencies
 for a series of scripts, this section must be included.
 
-The C<auth> section identifies the author in GitHub or other repository hosting
-site, such as Bitbucket or GitLab. This field is I<customary>, since it's used
-to identify the author in the ecosystem, and opens the possibility of having
-modules with the same name and different authors.
+Optionally, you can set an C<api> field. Incrementing this indicates
+that the interface provided by your module is not backwards compatible
+with a previous version. You can use it if you want to adhere to
+L<Semantic Versioning|https://semver.org>. A best practice is to simply
+keep the C<api> field to the same value as your major version number. A
+dependency can then depend on your module by including a C<:api> part,
+which will ensure backwards incompatible releases will not be pulled in.
+
+The C<auth> section identifies the author in GitHub or other repository
+hosting site, such as Bitbucket or GitLab. This field is I<customary>,
+since it's used to identify the author in the ecosystem, and opens the
+possibility of having modules with the same name and different authors.
 
 The C<authors> section includes a list of all the module authors. In
 the case there is only one author, a single element list must be