docs: document our API/ABI stability stance

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+The maintainers of the libgit2 project believe that having a stable API
+to program against is important for our users and the ecosystem - whether
+you're building against the libgit2 C APIs directly, creating a wrapper to
+a managed language, or programming against one of those managed wrappers
+like LibGit2Sharp or Rugged.
+
+Our API stability considerations are:
+
+* Our standard API is considered stable through a major release.
+
+  * We define our "standard API" to be anything included in the "git2.h"
+    header - in other words, anything defined in a header in the `git2`
+    directory.
+
+  * APIs will maintain their signature and will not be removed within a
+    major release, but new APIs may be added.
+
+  * Any APIs may be marked as deprecated within a major release, but will
+    not be removed until the next major release (at the earliest).  You
+    may define `GIT_DEPRECATE_HARD` to produce compiler warnings if you
+    target these deprecated APIs.
+
+  * We consider API compatibility to be against the C APIs.  That means
+    that we may use macros to keep API compatibility - for example, if we
+    rename a structure from `git_widget_options` to `git_foobar_options`
+    then we would `#define git_widget_options git_foobar_options` to retain
+    API compatibility.  Note that this does _not_ provide ABI compatibility.
+
+* Our systems API is only considered stable through a _minor_ release.
+
+  * We define our "systems API" to be anything included in the `git2/sys`
+    directory.  These are not "standard" APIs but are mechanisms to extend
+    libgit2 by adding new extensions - for example, a custom HTTPS transport,
+    TLS engine, or merge strategy.
+
+  * Additionally, the cmake options and the resulting constants that it
+    produces to be "systems API".
+
+  * Generally these mechanism are well defined and will not need significant
+    changes, but are considered a part of the library itself and may need
+
+  * Systems API changes will be noted specially within a release's changelog.
+
+* Our ABI is only considered stable through a _minor_ release.
+
+  * Our ABI consists of actual symbol names in the library, the function
+    signatures, and the actual layout of structures.  These are only
+    stable within minor releases, they are not stable within major releases
+    (yet).
+
+  * Since many FFIs use ABIs directly (for example, .NET P/Invoke or Rust),
+    this instability is unfortunate.
+
+  * In a future major release, we will begin providing ABI stability
+    throughout the major release cycle.
+
+  * ABI changes will be noted specially within a release's changelog.
+
+* Point releases are _generally_ only for bugfixes, and generally do _not_
+  include new features.  This means that point releases generally do _not_
+  include new APIs.  Point releases will never break API, systems API or
+  ABI compatibility.
+